Descriptive studies
A- Popular studies ( Correlational studies.B- Individual studies( Case report, case series, & cross -sectional.
Describe the pattern of disease occurrence in relation to variables such as person, place and time.
Person; who is getting the disease? & who is not? Person characteristics are: Age, sex, race, marital status & socio-economic status (education, occupation & income).
Place; where are the rates of disease highest or lowest?
Time; When does the disease occur commonly or rarely?
Data provided by descriptive studies are essential for;
a- Public health administrators {which group(s) or subgroup(s) are more or less affected by the disease}.b- Epidemiologists {identification of risk factor(s)}
1- Correlational study (Ecological study);
Describe the disease in the entire population in relation to the factor of interest such as age, sex, calendar time, utilization of health services, consumption of certain food or medication.etc. Ex: To describe mortality from coronary heart disease (CHD) in 5 countries (population) with per capita cigarette consumption in one year.SHAPE \* MERGEFORMAT
The descriptive measure of association in correlational study is ''correlation coefficient'' (r) which ranges from (-1) to(+1).If r = -ve → inverse association (may be preventive)
If r = +ve → +ve association (may be causal)
If r = 0 → No association.
*Advantages;
a- Quick & inexpensive. b- Use already available data.
c- Usually used as a first step in investigation a possible exposure- out come relation-ship.
*Limitations;
a- In ability to determine the temporal relation-ship between exposure & out come.b- Lack the ability to control for the effect of confounder.
c- Represent average exposure level rather actual individual level.
d- Formulate the hypothesis but can't test it.
# The presence of correlation doesn't necessarily imply the presence of a valid statistical association.
2- Case report study.
Describe the experience of a single patient. A condition develops in single individual and draws the attention of the clinician or the researcher. It is the first step in disease recognition. Ex; Kaposi sarcoma in healthy homosexual adult.3- Case series study.
Describe the experience of a group of patients with similar diagnosis.((Collection of individual case reports))
*Advantages (case report & case series) ;
a- Recognition of new disease e.g. AIDS.b- Formulation of hypothesis concerning possible risk factor.
*Limitations;
a- Based on experience of one or few patients only.
b- Lack of comparison group.
c- Formulate the hypothesis but can't test it.
4- Cross-Sectional Study (Prevalence Survey);
The exposure & the out come are assessed simultaneously among individual in well defined population. It is as if we were taking a photo-graph or a slice through the population at a point in time.
Ex; A researches wishes to investigate a possible association between cigarette smoking & CHD in a certain population, 146 young adults were randomly selected, smoking history was taken & an ECG performed for evidence of CHD.
Out-come(CHD) Total +ve -ve Exposure
(Smoking)+ve 13 73 86 -ve 2 58 60 Total 15 131 146*We can calculate the prevalence of disease (CHD) in person with exposure (Smoking) and compare it with the prevalence of disease in person with out exposure. Or compare the prevalence of exposure in person with disease to the prevalence of exposure in person with out the disease.
*Advantages.
1- Quick, inexpensive and less time consuming.2- Provide information about the frequency & characteristics of the disease.
3- Provide information about the prevalence of the disease.
*Limitations;
1- Not determine the temporal relation-ship between exposure & out-come.
2- Not determine prognostic factor from risk factor.
3- Liable for information bias (recall or interviewer bias).
4- Formulate the hypothesis but can't test it.
د. احمد Epidemiology General
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