Epidemiology study designs Dr PoonamNaik Professor Department of Community Medicine Yenepoya Medical College
Study design: Definition • A study design is a specific plan or protocol for conducting the study, which allows the investigator to translate the conceptual hypothesis (Research Question) into an operational one.
Selection of study design • Research Question • Exposure & Outcome • Intervention & Randomization • Cost • Feasibility • Ethical Issues
Quantitative designs Observational: Experimental: Studies that entail manipulation of the study factor (exposure) and randomization of subjects to treatment (exposure) groups • Studies that do not involve any intervention or experiment.
Descriptive Epidemiology • Distribution of disease is the domain of descriptive epidemiology. • Involves analysis of disease pattern in terms of • Generate Hypothesis. • Do not have comparison group. • Do not allow assessment of association. Time Person Place
Steps • Define the population • Define the disease under study • Describe the disease: Time, Place and person Time: Short term, Periodic and Long term 4. Measurement of disease 5. Compare with known indices 6. Formulate a hypothesis
Descriptive Epidemiology Advantages Disadvantages Temporal association between cause and effect is unclear. Causal inferences can not be drawn. Hypothesis can not be tested. • Easy availability of data. • Inexpensive • Provide data regarding magnitude of disease. • Few ethical difficulties. • Provide clues to disease etiology. • Formulation of hypothesis.
Case Reports • Case Report: Experience of a single Patient • An unusual disease or association. • Ex :- Benign hepatocellular adenomas, a rare tumour, in women who had taken oral contraceptives. • Confirmed a strong association.
Case Series • Case Series: • It aggregates individual cases in one report • Experience of a group of patients with similar diagnosis . • Constitute case group for case-control study.
Case Series • It is a Cross-sectional design • Information about causes (exposure variables) and effects (outcome variables) are collected at same point in time • Denominator population is not known • Cause-effect relationships (hypotheses) cannot be proved or rejected • New hypothesis can be generated
Case Reports and Case Series • Uses • To record unusual medical occurrences • In formulating useful hypothesis • Informative for rare clinical events • May suggest the emergence of new disease or epidemic • Limitations • Lack of appropriate comparison group
Design of Cross-sectional study.(A snapshot in time) Defined Population Gather Data on Exposure and Disease Exposed Do Not Have Disease Not Exposed Do Not Have Disease Exposed Have Disease Four Groups Are Possible Not Exposed Have disease Exposure Outcome
Examples • Study of Prevalence of hypertension among obese individuals residing in narketpally village. • Prevalence of Knee Osteoarthritis among premenopausal women in an urban Resettlement Colony in South Delhi.
Cross-sectional study Advantages Disadvantages Temporal sequence is often impossible. • Measure the exposure prevalence in relation to disease prevalence. • Suggest possible risk factor or risk factors for a disease.
Case Control Study( Research in reverse) • First step in searching for cause of an adverse health outcome. • Valuable when disease in question is rare. Case –Control study design:- Cases Controls Time
Retrospective study • Features: • Both exposure and outcome have occurred • Study proceeds backward from effect to cause • Uses a control group Look for: • If statistical association exists between a disease and a suspected factor • If it exists, the strength of association
Case Control Study With disease Without disease
Steps • Select cases: • Definition • Source: hospitals, population • Select controls: • Free from disease • Source: hospitals, relatives, neighborhood • Many cases, large study, cost - 1: 1 • Small: 2, 3 or 4 controls
Steps Matching: • Comparability • Remove confounding factor: associated with exposure, independent risk factor • Alcohol, Esophageal cancer: smoking • Suspected factor: not matched
Steps • Measure exposure: Interviews/questionnaires/past records • Analyze • Odds Ratio: Strength of association with risk factor
Analysis Calculate Exposure rates
Exposure Rates • Cases = a/ (a+c) = 33/35 = 94.2% • Controls = b/ (b+d) = 55/82 = 67.0% p < 0.001
Odds Ratio ( cross product ) Estimation of Risk • Odds Ratio =a d/b c = 8.1 • Measure of strength of association between risk factor and outcome. • Key parameter in the analysis of case control studies.
Bias Any systematic error in the determination of the association between the exposure and the disease.
Varieties of bias • Bias due to confounding • Memory or recall bias • Selection bias • Berkesonian bias: Different rates of admission to hospitals for people with different diseases • Interviewer’s bias
Advantages and Disadvantages • Easy to carry • Inexpensive, rapid • Few subjects • For rare diseases • No risk • Risk factors • No loss to follow-up • Bias: memory/past records • Appropriate controls • No incidence, only Odds ratio (estimate of risk)
Steps Select study subjects: Cohort • General population • Special groups Obtain data on exposure • Interviews • Medical examination/tests • Review of records
Continued….. Select comparison groups: • Internal comparison: degree of exposure • External comparison • General population Follow up: • Medical examination, review records • Loss to follow up
What to expect in analysis? • Relative risk:Incidence among Exposed/Unexposed • 2: twice higher • O.25: 75% reduction • Attributable risk: • (Incidence Exposed-Unexposed)/Incidence Exposed • What extent of the disease is attributed to exposure?
Advantages and Disadvantages • Incidence • Many outcomes: exposure • Estimate relative risk • Dose-response ratio • Large number • Long time • Funding • Study: alter behavior • Loss to follow up