1. Epidemiological study designs
Dr. Asif Rehman

2. Some Basic Measurement in Epidemiology
Ratio: Ratio measures a disease frequency. It expresses a relation in size b/w two random quantities. The numerator is not a component of the denominator. The numerator and denominator may involve an interval of time or may be instantaneous in time. It expresses in the form of: x : y or x/y Example: the ratio of WBCs relative to red cells is 1:600 or 1/600, meaning that for each white cells, there are 600 RBCs Example: The no of children with scabies at certain time The no of children with malnutrition at certain time

3. Some Basic Measurement in Epidemiology
Proportion:
A proportion is a ratio which indicates the relation in magnitude of a part of whole. The numerator is always included in the denominator. A proportion is usually is expressed as percentage
Example: The no of children with scabies at certain time X 100
Total no of children in the village at same time

4. Epidemiological study designs
The primary concern:
The primary concern of the epidemiologist is to study disease occurrence in people, who during the course of their lives are exposed to numerous factors and circumstances, some of which may have a role in disease etiology.
The epidemiologist employs carefully design research strategies to explore disease etiology.
The epidemiological studies can be classified as observational studies and experimental studies with further subdivisions:
(K Park, 20th edition)

5. Epidemiological study designs
All Studies Experimental Observational RCT Community Trail Descriptive Analytical Cross sectional Surveys Qualitative Case Control (Cross sectional survey) Cohort

6. Research Hierarchy

7. Epidemiological study designs
Epidemiological studies: Observational studies allows nature to take its own course; the investigator measures but don’t intervene. Descriptive studies are limited to a description of the occurrence of a disease in a population. An Analytical studies goes further by analyzing the relationship between health status and other variables. Experimental studies involve an active attempt to change a disease determinant or the progress of a disease.

8. Epidemiological study designs
Descriptive studies;
Descriptive studies are usually the 1st phase of an epidemiological investigation. These studies are concerned with observing the distribution of disease or health related characteristics in human population & identifying the characteristics with which the disease in question seems to be associated. Such studies basically ask the question:
When is the disease occurring? (Time distribution)
Where is it occurring? (Place distribution)
Who is getting the disease? (Person distribution)

9. Epidemiological study designs
Procedure involved in Descriptive study:
Defining the population to be studied
Defining the disease under study
Describing the disease (Time, Place, Person)
Measurement of disease
Comparing with known indices
Formulation of an etiological hypothesis
(K Park, 20th edition)

10. Epidemiological study designs
Cross sectional study:
Cross sectional study is the simplest form of an observational study.
It is based on a single examination of a cross section of a population at one point in time. And the result of which can be projected on the whole population provided the sampling has been done correctly.
These studies are more useful for chronic than short-lived diseases
It is also known as a prevalence study as they determine the burden of a disease in a population e.g National Health Survey of Pakistan on the prevalence of hypertension…

11. Epidemiological study designs
Cross sectional study:
A survey is a classical example of cross sectional study.
The hallmark of such a studies is that there is no follow up.
Cross sectional study is not suitable to study the association b/w exposure and outcome. It is difficult for a researcher to establish whether the exposure preceded the outcome or not.
Ideally the exposure should always precede the outcome.
(K Park, 20th edition)

12. Epidemiological study designs
Cross sectional study:
For example if the researcher is studying the association of uric acid and Hypertension, and on analysis finds that most of the Hypertensive patients have Hyperuricemia as well; here the researcher can’t say with confidence that Hyperuricemia is really an exposure/risk factor for hypertension (outcome); as Hyperuricemia can cause Hypertension and Hypertension is also a risk factor for Hyperuricemia. Hence, temporal association can’t be established in such studies.
Temporal association is one of the 1st criteria according to Hill’s criteria to confirm an association b/w exposure and outcome.
Temporal association simply means that there has to be a time period between exposure and outcome and that exposure should always precede the outcome.

13. Epidemiological study designs
Cross sectional study:
For instance in the previous example it has to be shown that a person had Hyperuricemia initially and then after a period of time developed hypertension.
Unfortunately in a cross sectional study the data is collected on Hyperuricemia and hypertension at the same time and can’t establish which came first, the “Chick” and “Egg” phenomenon.
Hence the cross sectional studies are useful for determining the prevalence of a disease, but not recommended if the researcher wants to study an association b/w an exposure and an outcome.

14. Epidemiological study designs
Cross sectional study:
In a cross sectional study, data on both the exposure and outcome are determined at same time. Hence 4 groups are made.
Exposed and have outcome
Exposed but don’t have outcome
Un exposed but have outcome
Un exposed without outcome

15. Epidemiological study designs
Reference population Sample Exposed Not exposed No Disease Disease No Disease Disease Estimate exposure and outcome at one point of time

16. Cross-sectional study
Advantages
Easy to perform
Prevalence/Frequency of the disease can be calculated
Inexpensive as compared to Analytical study
Useful for measuring the current health status & planning for some health services
Take lesser time as compared to Analytical studies
Researcher can generate hypothesis

17. Cross-sectional study
Disadvantages
Temporal association can’t be established
Hypothesis though can be generated but it is a weak hypothesis which needs to be tested by conducting further analytical studies.

18. Exercise
4th year MBBS class of 100 students (40 Females & 60 males). Among male, 20 of them wear glasses.
Calculate the gender ratio of 4th year MBBS class
Calculate the proportion of male students wearing glasses.

19. THANK YOU