Analytical Epidemiology

It is primarily designed to establish the causes of disease by investigating the association between exposure to a risk factor and the occurrence of disease.

• Objective of this study → Hypothesis testing.
• Analytical epidemiology has 4 types:
  • Ecological study
  • Cross-sectional study
  • Case-control study
  • Cohort study

Ecological Study

• Also known as correlational study.
• Type of analytical epidemiological study, which provides the least satisfactory type of evidence on causality.
• It is the least preferable analytical design.
• Units of study → population.
• Done in a small time frame, uses data which is already available.

Cross-Sectional Study

• It is the simplest form of analytical study.
• These studies are surveys that take place over a limited period.
• Based on the single examination of a cross-section of a population ‘at one point of time’.
• Results of the sample are projected to the whole population.
• This study provides prevalence of disease.
• It is a ‘snapshot’ of the population.
• More useful for chronic disease.
• Gives information about the distribution of disease, not its etiology.
• Unit of study → Individual.

Epidemiological Study Types

Case Control Study

Other Names:

• Case reference study
• Retrospective study
• TROHOC study
• Backward looking study
• Effect to cause study
• Outcome to exposure study
• Disease to risk factor study

→ Unit of study → Individual.

→ This study is not expensive.

Advantages:

• Useful to study rare disease.
• Easy to carry out.
• Rapid and inexpensive.
• No loss to follow-up/ Attrition.
• Requires few subjects.

→ This study gives information about many risk factors for a single disease.

Disadvantages:

• Recall bias and selection bias are present.
• Cannot measure incidence.

→ Strength of association in case control study → odds ratio (cross product ratio).

Cohort Studies

Other Names:

• Prospective study
• Forward-looking study
• Causes to effect study
• Exposure to outcome study
• Risk factor to disease study
• Incidence study
• Follow-up study

→ This study ends with the disease.

→ It determines a single risk factor affecting many diseases.

→ Units of study → Individual.

Types of Cohort Study:

a) Prospective Cohort Study

• Known as current cohort study or concurrent cohort study.
• Only exposure has occurred when the study has begun.

Examples:

• Framingham heart study.
• Doll and Hill’s prospective study on smoking and lung cancer.

b) Retrospective Cohort Study

• Known as historical cohort study or non-concurrent cohort study.
• It has combined advantages of both cohort study and case-control study.
• Both exposure as well as outcome have occurred when the study has begun.

Examples:

• Effects of fetal monitoring on neonatal deaths.
• PVC exposure and angiosarcoma of the liver.

c) Combined Prospective-Retrospective Cohort Study

• Known as ‘mixed cohort study’.

Strength of Association in Cohort Study:

→ It is evaluated by:

• Relative Risk (RR)
• Attributable Risk (AR)
• Population Attributable Risk (PAR)

Comparison of Study Designs

Epidemiological Studies Summary

Experimental Epidemiology

Randomized Controlled Trials (RCTs)

→ RCT is of two types:

Concurrent Parallel Design:

→ Comparisons are made between 2 groups:

• Experimental group: Is exposed to specific medication or intervention.
• Reference group: Is not exposed to specific medication or intervention.

Crossover Design:

→ Comparisons are made between 2 groups:

• Experimental group: Is exposed to specific medication or intervention.
• Reference group: Is not exposed to specific medication or intervention.

→ Then the groups are crossed-over (Exposed group now becomes non-exposed and vice-versa).

→ Crossover design RCT helps removing ethical concerns.

Intention to Treat Trial:

→ Implies that the results of an RCT are unaffected by attrition (Loss to follow-up) or changeover of study subjects from one group to another.

Randomization in RCT:

→ It is a statistical procedure, by which participants are allocated into either of two groups:

• Experimental group: In which intervention is given.
• Reference group: In which intervention is not given.

→ Randomization is best done by ‘Random number tables’.

Pre-Clinical & Clinical Trials