Labs

Study Designs Group Work

Epidemiological Studies
Descriptive studies
Analytic studies
Population
Individuals
Correlation Studies
-Case Report
-Case series
-Cross Section Studies
Observation Studies
Intervention studies
-Case Control Studies
-Cohort studies
Experimental studies (clinical trials)

The Five W’s of Journalism / Epidemiology

• What
• Who
• Where
• When
• Why / How
Clinical
Person
Place
Time


Cause,
mode of
transmission, risk factors

Descriptive Epidemiology

(Distribution)

Analytic

Epidemiology
(Determinants)

DESCRIPTIVE STUDIES

Advantages:
They use already available data
They are less expensive and less time consuming
They describe the pattern of disease occurrence
They formulate research hypothesis

I. Correlation or Ecological Studies

Based on aggregate measures of exposure and outcome from several populations.


The population is the unit of observation available for study.

II. Case report and case series

The individual is the unit of observation.
They describe the experience of a single patient or a small number of patients with a similar diagnosis reflecting unusual features of a disease.
They help in:
formulation of a hypothesis on etiological association
represent the first clues in identification of new disease or epidemic.

III. Cross sectional surveys

Collection of data on several individuals at “one point” in time.
Determines prevalence at a point in time
Therefore, CS is a prevalence study
The exposure and disease status are assessed simultaneously among individuals in a well defined population.
Snapshot in time


Cross Sectional Studies

• Disease

• Total

• Present

• Absent

• Exposure

• present
• a
• b
• a+b

• absent

• c
• d
• c+d
• Total


• a +c
• b+d
• N

Design of a C-S Study

CASE-CONTROL STUDIES

• Disease

• Total

• Present
• Absent

• Exposure

• Present
• a
• b
• a+b


• Absent
• c
• d
• c+d
• Total

• a +c

• b+d
• N

Case-Control Studies – Timing

Exposure

?
?
Exposed
Unexposed
Disease

Yes (case)

No (control)
Investigator


Strengths:
• Suitable for diseases of long latency period
• Quick and inexpensive (as compared to other analytic studies)
• 3. Suitable for rare diseases
• 4. Can examine multiple etiologic factors for a single outcome.
• 5. Requires fewer subjects at entry
• 6. Few ethical problems

Case-Control – 2-by-2 Table

• Case
• Exposed
• a
• Unexposed
• c
• V1
• Control
• b
• d
• V2
Odds Ratio = (a/c) / (b/d) = ad / bc


COHORT STUDIES

• Disease

• Total

• Present

• Absent

• Exposure

• present
• a
• b
• a+b

• absent

• c
• d
• c+d
• Total


• a +c
• b+d
• N

Strengths:

1. Establish the temporal relationship between exposure and disease
2. Useful for rare exposures
3. Examine multiple outcomes of single exposure
4. Allows direct measurement of incidence of the disease among exposed and unexposed groups.

Analysis

Incidence among exposed
Relative Risk (RR)=----------------------------------------
Incidence among non-exposed

a / a+b
RR =-------------
c/ c+d

Types of Cohort Studies:

1. Retrospective Cohort Study
2. Prospective Cohort Study
3. Ambidirectional Cohort Study


INTERVENTIONAL STUDIES“CLINICAL TRIALS”“Experimental Studies”

Investigator

Determines
Exposure Status

“CLINICAL TRIALS”

Strengths:
1. The gold standard of epidemiological studies.
2. Detect mild to moderate differences (10-20%)
3. Can control many confounding factors
4. Can demonstrate the temporal relationship with the highest degree of confidence
5. Randomized trial can yield the strongest and most direct epidemiological evidence to judge on causal association





Design of a
Randomized Clinical Trial

Selection of Subjects:Stratified Randomization

Analysis
Relative risk is the measure of association in clinical trials.

Group 1

In a study to determine the role of coffee consumption on risk of development of ovarian cancer, 188 patients with epithelial ovarian cancer were compared with 280 hospital controls and 259 health controls from the general population.
The results were as follows:

• Coffee drinking

• Cases
• Hospital Controls
• Population Controls
• Total
• Yes
• 177
• 249
• 233
• 659
• No
• 11
• 31
• 26
• 68
• Total
• 188
• 280
• 259
• 727


What is the design of this study?
Calculate the measure(s) of association
In your opinion, why they used two control groups?

Group 2

A research team wishes to investigate a possible association between smokless tobacco and oral lesions among professional football players. At spring training camp, they ask each player about current and past use of smokeless tobacco, cigarette and alcohol, and a dentist notes the type and extent of the lesions in the mouth.

• Mouth lesions

• No Lesions
• Total
• Smokless Tobacco
• Users
• 80
• 30
• 110

• Non Users

• 2
• 34
• 36
• Total


• 82
• 64
• 146

What is the design of this study?

Mention the single most important limitation of this study design.
Calculate the measure of morbidity in the entire group.

Group 3

100 individuals were inoculated with a vaccine against certain infectious disease, and 100 indistinguishable from the first group received a placebo.
During a subsequent epidemic, although the two groups had similar exposure, 20 of the vaccinated group, and 50 of the un-vaccinated group developed the disease.

What is the design of this study

Is the vaccine effective?
Calculate vaccine efficacy.

Group 4

An investigator wishes to study the relationship between bladder cancer and frozen yogurt. He gets cases of bladder cancer from the cancer registry and a control population of patients attending the urology clinic for reasons other than bladder cancer. He then interviews each patient to find out if they consume frozen yogurt.
Since smoking has also been shown to be associated with bladder cancer, he also classified cases and controls as either smokers or non-smokers. The results of the study are shown in the following table.

• Smokers

• cases

• control
• Yogurt Use
• Users
• 150
• 100

• Non users

• 50
• 200
• Non Smokers

• cases

• control
• Yogurt Use
• Users
• 200
• 100


• Non users
• 100
• 200

What is the design of this epidemiological study?

Calculate the measure of association among smokers, non smokers, and the whole group.
What do we call smoking in this example

Group 5

To assess the relationship between hypothyroidism and myocardial infarction, 1,000 women aged >55 years with hypothyroidism were compared with 3,000 women aged >55 years free from hypothyroidism.
All had no past history of myocardial infarction.
After a follow up period of 5 years, 40 females with subclinical hypothyroidism developed myocardial infarction (MI) and another 30 females with no evidence of subclinical hypothyroidism also developed MI.

Construct the suitable table

What is the design of this study?
Calculate the measure of association between Hypothyroidism and MI
Interpret the result.