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Is defined as the distribution and determinants of disease frequency or health events in man, and application to the improvement of health of populations (John M Last, 1988).
Distribution refers to the time, place and person characteristics of disease; (Study of disease distribution is a part of descriptive epidemiology) Determinants (what determines disease) are generally characterized as agent, host and environmental factors (Such studies form the Analytical Epidemiology).



Types of Epidemiological studies:-



1. RCTs           

1. Cohort Study

2. Field Trials

2. Case Control Study

3. Community Trials

3. Cross-sectional Study

4. Ecological Study

  1. Analytical studies


Also known as

Unit of study

Ecological studies

Correlation studies


Cross –sectional

Prevalence studies


Case control studies

Case reference studies


Cohort studies

Follow-up studies


  1. Experimental studies (interventional studies)


Also known as

Unit of study

Randomized Control trials

Clinical trials


Field trials

Community intervention studies

Healthy people

Community trials


  1. Analytical studies
    Analytical methods are concerned with the determinants of disease/health condition rather than with the distribution.
    They focus attention on ways to prove a hypothesis suggested by the earlier types of studies. The main types are:
    1. Case control study:
      1. ​An epidemiological study where a group of individuals with disease (Case) are compared with a group of individuals who are not suffering from disease (Controls) in terms of specific disease causing exposures.
      2. Since the starting point is a group of people who already have suffered from the disease, this is labeled as a retrospective study.(Backward looking Study).
      3. Advantages of case control studies.
        1. ​Relatively quick and easy to undertake.
        2. Relatively cheap to undertake.
        3. Only method useful in rare diseases.
        4. Not enmeshed in problems of follow-up as the data is collected at one point in time.
        5. Can be used to study the effect of many exposure variables on a single disease outcome.
      4. ​Drawbacks of case control studies
        1. ​Prone to selection and recall bias.
        2. Can’t measure relative risk or provide incidence estimates Q. ( Only odd’s ratio can be calculated from the type of study, which is a rough estimate of relative risk. But when the disease in question is a rare one odd’s ratio is almost equal to relative risk.)
        3. Sometimes the occurrence of the exposure in terms of time, i.e. whether it occurred before the disease may be difficult to estimate.
        4. Can’t be used for rare exposures.
      5. Cross over study: used when case serves his own control.
    2. ​​​Cohort study:
      The literal meaning of the term ‘cohort’ refers to a group that shares similar characteristics.
      1. Cohort studies are Forward Looking; look for the development of disease in a group of individuals (the cohort) free of the same at the beginning.
      2. The group is followed up over a period of time. During this period some persons will develop the disease under study while others will remain free of the disease (Follow-up Study).
      3. The characteristics (and exposure to disease causing factors) are compared between those who suffer from disease and those free from the disease.
      4. Thus, in cohort study, identified groups of populations who are free of the disease being studied and who are similar in all respects, except the specific exposure variable or characteristic whose effect is being related to the disease being studied. These groups are then followed up for the period of time that it takes for the disease to develop.
      5. A cohort study can either be prospective or retrospective but unless otherwise specified it is assumed to be prospective in design, but a case control study is always retrospective.
      6. Advantages of Cohort Study:
        • ​​Can calculate the incidence rate and hence relative risk can be computed(AI’09)
        • Temporality of association can be established
        • Can be used to study multiple outcomes of one type of exposure
      7. Types of cohort studies:
        • ​​Prospective
        • Retrospective
        • Mixed
  1. Variation  in  Cohort  studies:
    Before and after studies:-
    1. Also known as time series
    2. Effect is studied before and after intervention
    3. (AIIMS May’09)Nested Case Control studies: - it’s a type of Cohort study in which all those who develop the outcome of interest becomes the cases while those who are free becomes control. The advantage is that there is omission of confounders and the problem of recall bias is omitted out, last but not the least it is cost effective.
  2. Advantages of nested case control study
    1. Baseline data taken initially, no problems of recalling an event- minimization of bias.
    2. Samples are taken a priori before occurrence of the disease- temporality of the event can be easily established (In a traditional case control study we do not know that the events preceded the outcome or not).
    3. Cost reduced- All samples need not be analyzed.
    4. Deciding the direction of the study: the main determinant of the direction of study is the time when the outcome of interest occurs i.e whether the study begins with the outcome(retrospective) or the exposure which is thought to be causing the outcome(prospective).
  1. ​​Cross sectional studies:
    1. It examines the relationship between the disease and other characteristic of interest, as they exist in a defined population at a given time.
    2. Easy to carry out.
    3. If the population is followed for more than once it becomes longitudinal study.
    4. Multiple cross sectional studies may prove to be cost effective.
    5. They are commonly known as Prevaelance Study.

  2. Ecological or Correlational Study: Population is the Unit of Study.
    1. Bias in analytical studies:
      BIAS is any systematic error that occurs during any stage of the study, thereby resulting in mistaken estimate of exposure and/or outcome. A case control study is more prone to bias as compared to a cohort study. Although included as a bias confounding is not a systematic error in measurement, it is a true phenomenon existing in nature.
    2. Several types of bias can occur.
      1. Selection bias – occurs when the case and controls selected for the study are not representative of the general population, hence the results can’t be generalized (loss of external validity).
      2. Measurement bias – Occurs while data collection and occurs when the subjects can’t recall the events properly or the investigator is biased in collecting data because of prior knowledge of the association between the risk factor and the disease, e.g. if the investigator is studying the relationship between smoking and lung cancer he might take a more detailed questioning of the cases and not the controls and thus exaggerate the relationship.
      3. Confounding bias – Occurs when the disease under study is associated with another factor apart from the risk factor under study. In such a scenario it might be difficult to interpret the results, e.g. if we are studying the association between smoking and CHD; it is possible that the cases also take alcohol, thus confounding the results. This bias can be minimized by matching Q. Statistical procedures like regression analysis also help in detecting potential confounders.
      4. Berkesonian Bias – Differing rates of admissions to hospitals cause bias.
      5. Surveillance bias'- If a population is monitored-over a period of time, disease ascertainment may be better in the monitored population, than in the general population and may introduce a surveillance bias which leads to an erroneous estimate of RR or OR. E.g.-physicians monitored patients who had been prescribed OCPs much more closely than they monitored their other patients. As a result, They were more apt to identify cases of thrombophlebitis that developed in those patients who were taking OCPs (and who were therefore being more closely monitored) than among other patients who were not well monitored. As a result, just through better ascertainment of thrombophlebitis in women receiving OCPs, an apparent association of thrombophlebitis with OCPs may be observed, even if no true association occurs.
  1. Experimental studies
    1. Randomized Controlled Trial (AIIMS Nov’08)
      A randomized controlled trial is an experimental method where individuals are randomly
      allocated to an experimental or a control group and the effect or response of a drug or
      intervention is compared between the two groups.
      1. The two key features are ‘randomization’ and ‘controlled’. To
        ensure that there is no bias on part of the investigator, these trials are ‘blinded’
        – Blinding is done to Remove Measurement Bias.
        1. “Single” blind means the patients do not know what the intervention is.
        2. ‘Double’ blind means that neither the patient nor the researcher knows
          what the drug/intervention package is (MC Blinding design).
        3. ‘Triple’ blind means that the analysis team also does not know which
          is the experimental and which is the control group till the study has been
          completed.(Best Blinding Design).
      2. A unique strength of this type of study is that it eliminates confounding bias both
        known and unknown.
      3. Drawbacks of RCT:-
        1. External Validity that refers to the extent to which results can be generalized to
          a broader community.
        2. In RCT for including the persons in trial a screening test is done then only they
          are included in the trial and this may lead to that the volunteers selected in trial
          may be different from those who do not for e.g. Their health may be better.
        3. RCT cannot be used in some instances such as exposure to the harmful materials
          e.g. toxins bacteria etc.
    2. ​​Non randomized control trials:
      In some trials the participants are not randomly allocated to the exposures e.g. treatment or prevention strategies instead of using true random techniques the researcher uses alternate techniques e.g. Alternate allocation.
      The measurement of the out come is as same as the Cohort study.
      Drawback is chances of selection bias.
    3. Community Trial:
      A community trial is a modification of the clinical trial where instead of individuals being
      allocated to experimental or control groups, whole communities are randomly allocated to
      receive specific interventions and analysis is done for whole communities rather than for
      individuals in the communities. Vitamin A supplementation trials, iron supplementation
      or iodine supplementation trials are all examples of community trials.
    4. Vaccine trial
      1. When a randomized control trial is done to test the efficacy of a vaccine in preventing
        disease, it is called a vaccine trial.
      2. Double-blind randomized control trial is the most common type of experimental study
    5. Purpose of randomization in a Randomized Controlled trial (AIIMS Nov’08)
      1. If we randomize properly, we achieve non predictability of the next assignment, we
        do not have to worry that any subjective biases of the investigators, either overt or
        covert, may be introduced into the process of selecting patients for one treatment group or the other.
      2. Also, in long run, randomization will increase the likelihood that the groups will be
        comparable in regard to characteristics about which we may be concerned, such as sex,
        age, race and severity of disease, and that may affect prognosis. However, randomization
        is not a guarantee of comparability, because chance may playa role in the process, but
        over the long term, the groups will tend to be similar.
    6. Phases of Randomized Controlled Clinical Trials
      1. Phase I: Drug toxicity trials - done on "Healthy Volunteers" for side effects, tolerable dose
        of drug,  drug availability and bio metabolism. Typically requires about 20-80 subjects or
      2. Phase II: Initial clinical investigation for treatment effect - Done on patients + some healthy volunteers. Primary aim is to ratify the phase I findings, and also determine the dosage
        schedule and group of patients who are benefited by drug. Sample size 100 - 200.
      3. Phase III: Full Scale evaluation of treatment - Comparing with standard regimens and/or
        placebo synonymous with "Clinical Trials".
      4. Phase IV: Post Marketing Surveillance - Done after 'Drug' launched for large scale data on
        efficacy, safety, and long term side effects etc.
      5. Phase V: Re-efficacy trials. This is seldom done. Usually high degree of adverse effects from data in phase IV is the driving point for these.
    7. Sample size of a randomized trial is dependent on (AIIMS Nov’08)
      1. The difference in response rates (effect size) to be detected- higher the effect size, lower is the sample size required.
      2. Level of statistical significance - lower the value of p, greater the sample size required.
      3. Power of the study- increase the power, sample size increases.
      4. Non response rate- if higher, sample size required is high.
      5. The hypothesis should be stated- whether the test should be one sided or two sided.
    8. Cross-over type of trials
      1. Patients are randomly assigned to a study group and control group.
      2. The study group receives the treatment under consideration.
      3. The control group receives some alternate form of active treatment or placebo.
      4. The two groups are observed over time.
      5. Patients in each group are taken off their medication or placebo to allow for the elimination of the education from the body and for the possibility of any carry over effects- washout period.
      6. After this period, the two groups are switched- those who received the treatment under study are changed to the control group therapy or placebo, and vice versa.
  2. Descriptive studies
    Descriptive deals with the distribution of disease/health condition. There are a number of different descriptive methods:
    1. Case reports: Single  observation by a clinician which prompts further investigations with a more rigorous study design. Eg. Use of OCP as observed by one clinician that leads to Benign Ca was case reporting which later led to case control study with proven casual relationship between the same.
    2. Case series: A case series aggregates individual cases in one report. Sometimes several such cases with in a short period may lead to an alarm for an impending epidemic e.g. A cluster of homosexual mans in North America with similar sign sand symptoms had alarmed the world of a disease which today is known as AIDS.
    3. Cross sectional: discussed above.
    4. Surveillance: Its type of descriptive study. Defined as the ongoing systematic collection, analysis and interpretation of health data essential to the planning, implementation and evaluation of public health practices. Surveillance can be ACTIVE or Passive or Sentinel.
    5. Ecological Co relational studies: (AIIMS May’08)
      This is studies done to look the association between exposures and outcomes in population rather than the individuals. As much data has already been collected, these studies are suitable only for initial search of hypothesis.

The biggest drawback of such studies is that their inability to link exposures to outcome in individuals (this phenomenon is called ecological fallacy and is defined as ascribing to the members of the group characteristics that they in fact do not posses as individuals) and to control the confounders.

  1. Preference of Studies for establishing causation:
  1. Risk and its measurement:-
    Risk is measured to help to:-
    1. Prevention of disease.
    2. Predict incidence and prevalence of disease.
    3. To help diagnose the disease.
    4. To help to establish the cause of the disease of unknown etiology.
  2. Measures of Risk:-
    1. Absolute Risk
    2. Relative Risk
    3. Attributable Risk
    4. Odds Ratio
    5. Population Attributable Risk
  1. Absolute Risk is incidence of disease in the population. Eg. Incidence of Ca Lung.
  2. Relative Risk states by how many times exposure to any specific risk factors increases risk of contracting the disease.
    Estimated in a cohort study

  • It is direct measure of the Strength of association between the suspected cause and effect.
If RR=1 No Association
If RR>1 +ve Association
If RR<1 –ve Association
  1. Attributable Risk signifies to what extent the incidence of any disease could be attributed to the risk factor in question. (Also termed as Risk Difference).


AR helps in estimating to what extend will the incidence of any disease may decrease in a population if the causative agent is removed.
Estimated by help of cohort study.
  1. Odds Ratio:- Estimated by Case control study
    It is also known as cross product ratio obtained by multiplying the values in the diagonal cells in a 2x2 table.
    If OR=1 Then Exposure is not related to the Disease
    If OR <1 Exposure is negatively related to the Disease
    If OR is >1 Exposure is positively related to the Disease
  • If Disease is very rare RR=OR
  1. Risk Factors can be defined as the attribute or exposure that is significantly associated with the development of a disease. The main types are:-
    1. Causative eg. Smoking for ca lung
    2. Contributory eg. Lack of exercise for HT
    3. Predictive eg. Illiteracy for perinatal mortality
    4. Modifiable eg alcoholism, obesity, sedentary life style for HT
    5. Un modifiable eg. Age, Sex, Race
  2. Criteria for Causal Association: (AIIMS May’08)
    1. Strength of association
    2. Temporality of association
    3. Biological gradient: dose and duration response relationship
    4. Consistency of association
    5. Specificity of association
    6. Biological plausibility
    7. Cessation Experiment
    8. Coherence
When most if not all of these criteria are met only then an association is said to be causal.

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