بسم الله الرحمن الرحيم
By assistant prof. dr. Ali Abid Saadoon /Thiqar university / Medical college/ community medicine department
By professor dr. Ali Abid Saadoon /university of Thi-Qar / college of Medicine / community medicine department
Introduction to descriptive Epidemiology
Concepts, methods and statisticsِ
Objectives
1. Describe tools and indicators used in population surveys for describing health or disease in a community and assess the appropriateness of each in given setting.2. Interpret the distribution of disease in a population according to person, place and time.
3. List and define various rates used in epidemiological settings and explain their use for comparative purposes.
4. Describe, using examples, the formulation and testing of epidemiological hypothesis
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5. Identify steps involved in planning, conduct and interpretation of analytical epidemiological studies, to establish determinants of disease and other health-related phenomena.6. Define the concepts of risk, risk factor, relative risk and attributable risk and interpret their use
7. Define the concept "cohort" and recognize a cohort effect when interpreting cross-sectional data.
8. Calculate indices used in the estimation of the validity of tests commonly used in epidemiological studies.
9. Describe the epidemiological methods used by health administrators in planning, organization and evaluation of health care programmes in different contexts.
10. List and critically examine the steps applicable in the investigation and management of epidemics.
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11. Examine and criticize the effectiveness and efficiency of therapeutic and preventive measures based upon results of randomized clinical trials.12. Determine the role of epidemiological methods in the diagnosis, management and prognosis of disease in clinical settings.
13. Define and distinguish association and causation and list epidemiological criteria for causal association.
THE CONCEPT OF HEALTH AND DISEASE
Health: The concept health may mean different things for different people. Health may simply mean the absence of disease or it may mean the soundness of body. The World Health Organization (WHO) defined health as “A state of complete physical, mental, and social wellbeingand not merely the absence of disease or infirmity.
The main problem with this definition is the lack of agreement on the meaning of the term “complete” which remained difficult to quantify but encouraged almost all countries to work towards the improvement of the health of their people. A more practical definition of health may be as follows:
Health is a state of successful adaptation of the body to stresses and stimuli to which it is subjected. The successful adaptation may be very optimal and the attributes of optimal health are:
1. Anatomical integrity to ensure the physical aspect of health
2. Ability to do normal duties at personal, family and community level
3. Ability to deal with stress whether this stress is physical, mental or social
4. Feeling of well being. This is the mental or psychological dimension of health
5. Freedom from disease and premature death
6. Spiritual and moral stability
In this sense, health is a changing state encompassing a spectrum extending from very optimal health at one extreme point to just a point when no element of health does exist and death is the other end of the spectrum. Health has many dimensions:
1. Physical dimension
2. Mental dimension3. Social dimension
4. Spiritual and ethical dimension
5. Emotional dimension
6. Vocational dimension
7. Other dimensions, philosophical, socioeconomic, medical, environmental
Optimal health--------------------------------------Death
Disease (Dis- ease) is failure of the adaptive mechanism of the body to overcome external stress and stimuli to which it is exposed, resulting in abnormal structure and/ or function of one or more of its tissues, organs, or systems.
Such abnormality is either reversible or irreversible acute or chronic.
The Risk of disease is not the same for every individual in any given population. It is also differ from one population to another at one time and in the same population at different times.
To give examples, the risk of diabetes mellitus is not the same for every individual in the population of Thi-Qar, otherwise everyone should develop this disease at a given age and this age is common to all those affected. The situation in real life is not so. Many people spend their lives without diabetes and the age at which some people develop this disease is very variable. This variation in risk as well as the variation in age of onset of the disease must reflect variation in the exposure and susceptibility of individuals. The same argument may be used to explain the variation in the risk of disease in different populations. It is definite that the prevalence of hypertension is higher in many industrialized countries than in some underdeveloped countries
The risk factors and the causes of ill health (disease) are generally operating in the environment where people are living.
Environmental situations are not the same for different communities. In addition, people are different from genetic point of view. Therefore, the risk of disease is not the same for population with different environmental conditions. That is why, the distribution of disease is not random among individuals and among populations, and that is why doctors working within the walls of hospitals cannot understand the circumstances surrounding the process of disease and consequently they cannot modify these circumstances. Under the best of their work, they may modify the outcome of ill effect of environment.
Sometimes, they do add some disease to the patients who are treated at health care institutions (the so called iatrogenic disease).
ECOLOGY OF HEALTH
Ecology of healthis the study of all factors in the environment that affects the health of man. Any given state of health is a product of interaction (visible and invisible) of enormous number of factors.
These are conventionally grouped into:
Factors related to host
Such as age, sex, occupation, education, genetics, habits, and other personal characteristics. Smoking for example is an important contributor in the etiology of lung cancer and ischemic heart diseases. Many diseases are genetically determined such as sickle cell disease and thalassaemia.Factors related to disease agent
like, type of agent (bacteria, viruses, parasites, fungi etc.) dose and duration of exposure, virulence, and susceptibility to damaging effects of the environment such as heat and light.Factors related to environment
in its physical, chemical, biological, and social dimensions. Environment:1. assists the survival of disease agent
2. facilitates the transmission of disease agent from the source to a new host and
3. affects the immunity of the host. See the diagram below:
1 Environment
2 3
Agent Host
THE CONCEPT OF PREVENTION
Prevention is to make the occurrence of some thing like disease, accident, which is anticipated (predictable), impossible. This can be achieved at four levels (levels of prevention):Primordial prevention
It is primary prevention made early in life (childhood). The main intervention is through individual and mass education to discourage harmful lifestyles.Primary prevention
: All measures those are applicable before the onset of disease through health promotion and specific protection. Examples are good housing, regular exercise, immunization, personal hygiene, the use of safety beltSecondary prevention
This is applied after the onset of disease through early detection and prompt treatment of disease. Early detection of disease may be achieved through contact tracing , periodical medical examination, pre-employment examination, and surveillance. Secondary prevention may reduce the length of illness, the length of infectiousness, the risk of complications, and the economic losses by the individuals, their families, and the society at large.Tertiary prevention:
through limitation of disability and rehabilitation. Rehabilitation may be physical, educational, social, or vocational.Importance
Epidemiology is very helpful in understanding the complexities of health and disease and their respective determinants at population groups within an ecological context.Definition
Epidemiology is the study of frequency, distribution and determinants of disease and health related events in population groups. It helps in obtaining, interpretation and use of data to improve, protect and restore the health of the population.
Epidemiology is thus concerned with:
The frequency of events (how many)The distribution the event (pattern) and
The determinants of events (risk factors and causes of disease).
Phases/methods
Descriptive epidemiology in which we ask the following questions:What is the problem? How frequent is it?
Who are the people involved?Where does it take place?
When does it take place?In this manner, a problem, a disease or any health related event is identified and described in terms of:
characteristics of persons ( Who)
characteristics of place (Where )
characteristics of time ( When)Methods of epidemiology that are used to measure the frequency of disease and its distribution are called descriptive methods.
Analytical epidemiology:
In which we attempt to analyze the causes or determinants of disease by testing hypotheses to answer such questions as:- How is the disease caused?
- Why is it continuing?-Why is it affecting some persons rather than others?
Methods of epidemiology which test hypotheses are called analytical methods.
Interventional or experimental epidemiology:Clinical and community trials are used to test the effectiveness of new drugs or methods for controlling disease or improving underlying conditions.
Active efforts to interfere in the disease process and measure the effectiveness of such intervention in changing the incidence, severity of disease or recovery from it.
A situation-Intervention- Change- Measurement
Evaluative epidemiology:
- Recent development in the field of epidemiological methods.- The effectiveness of different health care programmes is evaluated
- Evaluative epidemiology is not necessarily interventional in nature.How various phases are related to each other?
Descriptive studiesData aggregation and analysis
Analysis of results Model building &
& further studies Formulation of hypothesisAnalytical studies to test
hypothesis
Scope of epidemiology
Actually, epidemiology has a widened scope nowadays to encompass such disciplines as:- Infectious disease epidemiology
- Nutritional epidemiology- Cancer epidemiology
- Cardiovascular epidemiology
- Neuroepidemiology
- Epidemiology of ageing- Psychiatric epidemiology
- Genetic epidemiology
- Pharmacoepidemiology
- Dermatoepidemiology
- Social epidemiology
- Epidemiology of health care
Purposes of epidemiology
• Community diagnosis:
What are the current and potential health problems of the community or population?
2. Historical or time trends.
Is the health of the population or community improving, deteriorating or unchanging with the passage of time?
• Individual risks and chances.
To estimate incidence rates and relative risks.4. Identification of aetiological factors.
5. Description of the natural history of disease:The understanding of such history is essential for screening and early intervention.
6. Classification of disease on the basis of causes (aetiological) or on the basis of features (manifestation).
7. Clinical decision analysis-clinical epidemiology.
8. Administrative uses- planning, organization and evaluation of promotive, protective, curative or rehabilitative health care services.
In achieving their purposes, epidemiologists make use of:
1. Basic biomedical and clinical sciences.
2. Vital statistics and biostatistics.3. Social sciences such as demography and sociology.
4. Environmental sciences such as physics and chemistry.Some of the contributions of epidemiology
Contribution to medical knowledge in communicable, nutritional and occupational diseases, in addition to recent contribution in almost all fields of health related aspects. Cholera, scurvey, thalidomide, AIDS of dilemmas in the field of health and disease.Contribution to health services.
Epidemiology is the basic discipline for:1. Diagnosis of population health problems.
2. Estimation of population health needs.
3. Determination of priorities.
4. Evaluation of services performance.
5. Basic science to evidence based medicine (EBM).
Contribution to medical education.
The curriculum of medical and health personnel is determined by many factors including pattern of disease.
Local contribution:
During the last 30 years, epidemiological methods became increasingly familiar to local physicians and researchers. Many research works were carried out in Basrah and other parts of Iraq. These were very useful in determining health indicators, in identifying risk factors of important diseases and in evaluation of the performance of health care services.Types and sources of epidemiological data
Types1. Data related to demographic characteristics of the population.
2. Data related to morbidity and mortality.
3. Data related to the level and distribution of available health care services.
4. Data related to the pattern and determinants of health care services utilization
5. Data related to indicators of health care impact
Sources of epidemiological data
1. Routine statistics sources2. Household surveys
3. Population censuses
4. Publications5. Electronic sources: Internet and Medline
Methods of data collectiona. Direct observation of events or states including physical examination.
b. Mail questionnaire.
c. Interviewing directly or through telephone and other communication media...
d. Extraction from records.
Routine statistical sources:
Routinely collected and related to the health and life of individuals and include:A. Vital statistics such as:
Birth certificates
Death certificates
Marriage certificates
Divorce certificates
B. Health statistics
These are statistics maintained by health care institutions
On different health related events which they deal with. Examples include:
• Outpatient and inpatient statistics
• Laboratory statistics
• Disease notification and surveillance statistics
• Special statistics related to cancer, tuberculosis and other disease registries
• Routine and periodic publications and reports
Information based on routine statistics is relatively useful for ongoing research, supervision, and evaluation of health care services. They are relatively easy to obtain and cheaper to use but they have two major limitations (disadvantages):
They are almost always incomplete.
They may have gross errors.
Epidemiological surveys (household surveys)
These surveys are specially designed and carried out for answering specific questions.
They include different types of epidemiological studies but two main types are commonly used.
Cross-sectional surveys or studies
These are based on a single observation usually carried out in a short time. They are characterized by the following:a. They usually measure prevalence of disease or related outcome.
b. They suggest hypotheses.
c. They are not useful for diseases of short duration. A single observation may miss cases.
d. Their results are difficult to interpret because of seasonal variation and cohort effect.
e. They are relatively quicker and cheaper to do.
f. Although they are essentially descriptive, they can be modified to estimate incidence of disease and to test hypotheses.
A case-control design can be made within the context of a cross-sectional study.
Longitudinal or follow up surveys or studies:
These are based on repeated observation of the study population over a defined period of time. They start with a base-line data provided by initial cross-sectional study.a. They measure incidence of disease or related outcome.
b. They suggest hypotheses.
c. They are relatively more expensive and difficult to organize.
d. They are not useful for diseases of rare occurrence.
e. The results are easier to interpret.
f. They can be useful to determine seasonal variation of disease and other health related outcomes.
Advantages of epidemiological (household) survey
a. They serve group diagnostic purposes.b. They identify population selective action in response to illness.
c. They help in assessing total population health needs.
d. They are particularly useful in rural areas where other data sources may not be available.
e. They are useful in evaluating health care effectiveness and in planning health care for population.
f. They identify services non-users.
g. They help in identifying factors, which determine utilization variation.
Problems associated with household surveys
a. Diagnostic accuracy: Diagnosis is based on population reporting which is technically questionable
b. Memory errors: People tend to remember important events
c. Over and under-reporting and non-response
d. Social distance between interviewer and interviewee
e. Sex of interviewer
f. Self-reporting and reporting for others
g. Cost and time required to achieve the work
Measurements of frequency of disease
Rates are used to express the frequency of an event (sickness, disease, birth, death… etc) per unit of size of related population. Time period and place are specified. All rates have:1. Numerator: cases or events
2. Denominator: population at risk
3. Time limit or reference period
4. A multiplication factor, usually a multiple of 10.
Population at risk: are those individuals who are at risk of getting ill and thus contributing to the cases (they became ill or diseased or die or give birth to live babies) which form the numerator. Generally the numerator is part of the denominator though this is not always true.
No. of persons with a characteristic or No. of
events during a specified period of time place
A rate =--------------------------------------------------------------- x K
population at risk
(Note: K is a multiplication factor)
Proportionsexpress the part (persons affected, number of cases or deaths of a given disease) in relation to the total persons, cases or deaths due to all diseases. The numerator is part of the denominator but there is no multiplication factor. The value of a proportion is usually less than unity (less than one) . It equals one only if all individuals at risk become diseased.
Ratios
express the number of persons with a characteristics relative to the number of persons without the characteristics. The numerator is not part of the denominator. Ratio is not a common epidemiological parameter in descriptive epidemiology. In analytical epidemiology, certain ratios aresignificant like SMR/Sir/RR/OR
In a village, there were 6000 persons. During the year 2001, a total of 240 live births took place, of whom 115 were female births. Use these data to measure frequencies of births as events in this population as follows:
Definition of basic rates
1. Rates related to fertility. These are useful indicators in health and demographic characteristics of population. These rates will be discussed in detail in the chapter on demography. The rates include:• Crude birth rate (CBR
No. of live births in a year
CBR = ------------------------------------------ x 1000Mid-year population
b. General fertility rate (GFR:
No. of live births in a yearGFR = ------------------------------------------------- x 1000
No. of women aged ( 15 – 49) years
c. Marital specific fertility rate.
No. of live births in a yearMSFR=-------------------------------------------------- x 1000
No. of married women aged (15-49) years
d. Total fertility rate (TFR): This refers to the total births a woman has during her full reproductive life. It equals to the summation of age - specific fertility rates when based on cross-sectional studies.
2. Rates related to morbidity:
Incidence rate: Incidence of a disease is the number of new cases or episodes of disease which occur during a specified period of time in a specific population or place.The incidence rate (IR) is the number of new cases or episodes (spells) of disease per unit of size of population.
Number of new cases of a disease in a year
in a given population
IR = -------------------------------------------------------------- X 1000
Total population at risk in the same year
Incidence rate is more useful in the following situations:
To study disease of short duration.To study the aetiology of disease.
To evaluate preventive measures.
To determine the risk of acquiring of disease.
To assess transmission of infectious agent.
Prevalence rate:
Number of existing cases (new & old) in a given population at a point in time
PnPR= ------------------------------------------------------- X 1000
Population at risk
Period prevalence rate (PrPP): Less commonly used .
Prevalence rates are useful for:1. Diseases of long duration.
2. Administrative purposes (e.g., planning of health services).
Relationship of incidence and prevalence
Point prevalence is a function of incidence , deaths and recovery of disease.Prevalence = Incidence X Duration
3. Rates related to mortality:1. Perinatal mortality rate.
2. Stillbirth rate.
3. Neonatal mortality rate (early and late).
4. Post neonatal mortality rate.
5. Infant mortality rate
6. Crude death rate.
7. Case fatality rate (ratio).
8. Maternal mortality rate.9. Specific mortality rates like age specific, sex specific, cause specific.
10. Proportional mortality ratio.
Fertility rate importan
that the fertility level in any population is affected by a complex of factors including the age at marriage, the proportion of women married, the pattern of contraception practice, the practice of abortion, the pattern of infant feeding (breast feeding or not ) and others. For your information we list the following figures for Iraq:CBR is from 30-45 per 1000 population, CDR is from 5-9 per 1000,
GFR is about five times the CBR. The TFR is somewhere between 4-7 per woman..
The following table shows the population of Basrah by sex and age, total cancer cases by sex and age and annual incidence rates for the year 2005
Age (yrs)
Males
Population Cases IR
Females
Population Cases IR
<5
5-14
15-44
45-64
65&over
138790 18 13.0
285112 49 17.2
505670 159 31.4
115121 301 261.5
31201 206 660.2
136354 24 17.6
277037 29 10.5
524855 286 54.5
106054 383 361.1
37876 148 390.7
Total
1 075 894 733 68.1
1 082 176 871 80.5
A longitudinal household survey was carried out in 1999 by a team of epidemiologists to generate relevant data on certain aspects of the health status of a given population, the following results were reported:
Total population 50 000
Annual live births 1800
Annual total deaths 300
Annual infant deaths 80
Number of new cases of pneumonia 64
Number of deaths due to pneumonia 2
Number of infants who received BCG 1650
4. Other useful rates:
A number of other useful rates are commonly used in the field of health care epidemiology. Examples are utilization rates, coverage rates, performance rates, bed occupancy rates., adequacy rates, proficiency rates…. etc.Efficacy
is the ability to get a job done satisfactorily. The word comes from the same roots as effectiveness, and it has often been used synonymously, although in pharmacology a distinction is now often made between efficacy and effectiveness. The word efficacy is used in pharmacology and medicine to refer to both the maximum response achievable from a pharmaceutical drug in research settings, and to the capacity for sufficient therapeutic effect or beneficial change in clinical settings.The concept of person, place and time
Three groups of variables are commonly used in descriptive epidemiology. These are:A. Characteristics describing the persons affected such as age, sex, marital status, education, occupation, habits, genetics and ethnic groups.
B. Characteristics describing the place where persons were found affected. The distribution of the disease may have:
International
National (limited to one country
Continental
Local: only part of a country or
Urban-rural pattern?
C. Characteristics describing the time in which persons were found affected. Does the distribution follows
Secular trend (over many years and decades)?
Seasonal trend (within the same year)? Recurrent pattern or the occurrence of disease after special events, e.g., raining. ?
Distribution of disease with age
It is well known that the distribution of disease is very variable with age. Actually, age is an important confounding variable and must be considered and controlled for (there are many ways to control for the effect of age) when the distribution of disease is examined in relation to other variables. The variation of disease distribution with age may be explained as follows1. Accuracy of diagnosis. Disease is less likely to be ascertained in extreme age groups. This leads to under estimation of certain causes of death in the very young and the very elderly people. In some instances, basic population data are lacking on such extreme age groups.
2. Variation in intensity and duration of exposure to risk factors.
3. Variation in immunity and susceptibility. People are not constant in their immunological response and immunological status and are not necessarily similar to each other in that respect
4. The type of epidemiological parameter used, i.e., incidence. Prevalence or
mortality. For example, in a disease with constant incidence with age andnegligible mortality and incomplete recovery, the age specific prevalence
increases with age because cases once developed tend to accumulate over time thus raising the prevalence
5. Bimodality. In some instances the distribution of disease frequency with age may have more than one peak (bimodal) as in case of the incidence of lymphomas leukaemias, testicular cancer and tuberculosis with age. This bimodality may suggest the heterogeneity of data and the possibility that we are dealing with two disease entities rather than with one disease. For example, the first peak in the incidence rate of tuberculosis in young children is definitely primary(exogenous) tuberculosis. On the other hand, the peak late in life is mainly secondary (endogenous) tuberculosis
6. Ageing or biological clock. Some times, people become very aging and lose the ability to carry out even simple tasks, yet they have no apparent disease. They probably follow a precoded biological clock, which determines the life span.
Distribution of disease with marital status
In many studies it was reported that death rates and suicidal rates are higher among non-married people (single, widowed and divorced) than they are in married people. This is true for both males and females. Such variation might be difficult to explain but two explanations are possible:
a. Marriage stabilizes life and reduces the risk of exposure to hazardous behaviour. Married people may feel more responsible not only for their lives but for the care and life of their spouses and children. They may avoid certain risky behaviours.
People who are unmarried are actually not healthy to start with and they prefer not to marry. The higher risk of death and suicide among them is perhaps related to their poor health to start with rather than marital status (being unmarried). But this is not completely true – at least not in some countries such as Germany, where it was shown, that widowed wives live considerably longer than married wives while the opposite effect was observed for males.
Interpretation of association of disease distribution with place
The following criteria are essential to demonstrate an association of disease distribution with place:1. High frequency rates of the disease are observed in all ethnic groups living in that place.
2. Similar people who inhabit other places do not show high frequency rates of the disease.
3. Healthy people entering the place become affected by the disease at a rate similar to that of the indigenous population.
4. People who leave the place and move to other places do not experience high frequency rates of the disease.
5. Species other than man may show similar pattern of the disease
When these criteria are fulfilled, it is possible to imply that:
Either the inhabitants of that particular place posses characteristics of importance to the aetiology of the disease.Or that the physical, chemical, biological or social environment of that place contain aetiological factors of the disease. More intensive research is required to identify such factors
Interpretation of disease variation with timeSecular changes
Distribution of disease with time may follow long term changes (secular changes or trends). The changes occur over years or decades. Examples are the changes in cancer, cardiovascular disease and AIDS. Such secular changes which show a clear rise in the disease frequency with time (years or even decades) as has been shown with the rise in mortality rate due to lung cancer in some European countries during the twentieth century could be explained as follows:1. The rise indicates real increase in the incidence of the disease in response to:
a. Massive exposure to disease agents.
b. Change in life style of the people and
c. Failure of adaptation to social change.
2. The rise is artificial due to
a. Improved diagnosis of disease.b. Change in classification of disease.
c. Improved recording of cases.
d. Ageing of the population/ change in population at risk
Seasonal changes
A change of disease frequency within the year reflects a change in population immunity (susceptibility), change in the environmental situation in favour of disease agent development or multiplication, and its transmission to new host or both.Epidemiological studies
Classification of Epidemiological studies1. Observational:
a. Descriptive- observe and describe. No controls no intervention.b. Analytical- observe (measure) and interpret. Controls are used but no intervention.
2. Interventional (experimental)- Interfere, observes and analyze (interpret)
3. Evaluational: Can use combination of observational descriptive, analytical and interventional approaches. Observational descriptive studies
1. Observational descriptive epidemiological studies(surveys or household surveys)
These surveys are specially designed and carried out for answering specific questions. They include different types of epidemiological studies but two main types are commonly used.Cross-sectional surveys or studies.
These are based on a single observation usually carried out in a short time and characterized by :a. They usually measure prevalence of disease
b. Based on aggregated evidence, they suggest hypotheses.
c. They are not useful for diseases of short duration. A single observation may miss cases.
d. Their results are difficult to interpret because of seasonal variation and cohort effect.
e. They are relatively quicker and cheaper to do compared to follow up studies.
f. They can be modified to estimate incidence of disease and to test hypotheses. A case-control design can be made within the context of a cross-sectional study.
Longitudinal or follow up surveys or studies.
These are based on repeated observation of the study population over a defined period of time. They start with a base-line data provided by initial cross-sectional study.a. They measure incidence of disease or related outcome.
b. Based on aggregated evidence, they suggest hypotheses.c. They are relatively more expensive and difficult to organize.
d. They are not useful for diseases of rare occurrence.
e. The results are easier to interpret.
f. They can be useful to determine seasonal variation of disease.
Advantages of epidemiological (household) surveys
Problems associated with household surveys
2. Observational analytical studies
Definition of basic termsRisk: A probability that an individual will become ill or die within a specified period of time or age. It is used to denote incidence rate (risk of acquiring disease) or mortality rate (risk of dying).
Risk factor: It can be defined as:
a. Risk marker. An attribute or an exposure that is associated with an increased risk of disease or other specific outcome.b. Determinant. An attribute or exposure that increases the risk of disease or other specific outcome.
c. Modifiable risk factor.
d. Non-modifiable risk factor:Relative risk or Risk Ratio (RR): is a measure of strength of association between an exposure (risk factor) and an outcome (disease).
Incidence rate among exposed
Relative risk (RR) = ----------------------------------------------------Incidence rate among non exposed
RR =1
RR >1
RR<1
Attributable risk (AR): It refers to the fraction of the incidence rate of the disease that can be attributed to the exposure to the risk factor. It is calculated by the following formula:
Attributable risk (AR)= IR among exposed – IR among non exposed
IR among exposed – IR among non exposed
% reduction = ------------------------------------------------------X 100IR among exposed
ANALYTICAL STUDIES
1. They test hypothesis2. They help in determination of risk factors (causes)
3. They involve the use of comparison or control groups
4. They need sound study design and high epidemiological expertise.
COHORT STUDIES
A cohort is a group of individuals who share common characteristics or experience, e.g., birth cohort which represents all live births in one year.In cohort studies
1. Select people who are free from the disease.2. At least two groups are used (exposed versus non- exposed)
3. The two groups are followed up for a period of time)
4. Events (new cases or deaths ) are recorded.
5. Results are analyzed to test the hypothesis.
Example
A study was carried out to ascertain the relationship of parental smoking to the risk of ARI among children aged <5 years. A total of 800 children of smoking parents and 1200 of nonsmoking parents were followed up for six months. During the follow up period, 592 of the first group and 636 of the second group developed at least one attack of ARI.
• Tabulate the data
• Calculate the incidence rates• Calculate the relative risk
• Calculate the attributable risk
• Perform a statistical test.
Note: In case of multiple exposures (the disease is related to multiple risk factors) a more sophisticated analysis is carried out to determine the relative effect or contribution of each risk factor. Logistic regression analysis and stepwise multiple regression analyses are commonly used. Computerized statistical packages (such as SPSS) are available for such sophisticated analyses.
CASE-CONTROL STUDIES
In case-control studies:1. Both exposure and outcome or disease have occurred before the start of the study.
2. The study proceeds backwards from outcome to cause (retrospective).
3. Controls are used to support or refute any inference.
The basic design steps
1. Selection of cases (persons with definite disease) and controls (persons definitely free from the disease at the time of the study).2. Matching for known confounding variables (at least age and sex).
3. Measurement of past exposure in both groups.
4. Analysis and interpretation.
Example
To illustrate the study design, we identify a number of children who are suffering from ARI (say pneumonia). Suppose we identified 240 with ARI 380 of children matched for age and sex but free from ARI. Suppose we found that the parents of 170 cases and 200 controls were smokers.
The analysis and interpretation
1. Tabulation of the data• Total
• Children without pneumonia
• Children with pneumonia
• History of smoking
• 370
• 200
• 170
• Positive
• 250
• 180
• 70
• Negative
• 620
• 380
• 240
• Total
2. Calculation of the % of smokers (exposed) among parents of cases and controls.
3. Calculation of the % of smokers among parents of controls
4. Measurement of the strength of association between parental smoking and acute respiratory infection. This is achieved by calculating a proxy measure to the relative risk. This measure is called the Odds ratio.
Cases exposed X Controls not exposed
The Odds Ratio =----------------------------------------------------------------------
Cases not exposed X Controls exposed
5. Perform a suitable statistical test to ascertain any significant association.
6. Calculation of the attributable riskb ( r - 1)
Attributable risk = -----------------------
b ( r – 1 ) + 1
Where
r = Odds ratiob = the proportion of people in the general population with the risk factor.
SOURCES OF CONTROLS IN ANALTICAL STUDIES
In case control studies, the main sources are:1. The total population in a given area, on the assumption that we know the extent of exposure in the general population. Otherwise, a population-based sample of controls can be drawn. This is the best source of controls but probably difficult in logistics terms.
2. Relatives and neighbours. This is useful to control for genetics and immediate environment
3. Hospital patients other than those with the disease under study.
4. Associates of cases in place of residence, schools, place of work.
In cohort studies the main sources are:
1. Built in comparative cohorts as for example in studying the relationship of lung cancer to smoking, people may be categorized into subgroups of heavy smokers, moderate smokers, light smokers and nonsmokers. Thus we have four (heavy, moderate, light and non) instead of just two (smokers versus non-smokers).2. Relatives and neighbors.
3. The total population provided that the level of exposure is ascertained at population level at the start of the study.4. Special occupational groups.
COMPARISON OF CASES CONTROL AND COHORT STUDIES
EndVery thanks for your good listening
