Lecture 7 Objective 18. Describe the elements of design of observational studies: case ‑ control studies (retrospective studies). Discuss the advantages and disadvantages of this design.

Retrospective Case Control Studies Case-control studies select cases (with disease) and controls (without disease of interest) sometimes matched on potentially confounding variables

Retrospective Case Control Studies presence or absence of the risk factor is determined for each individual. This is the most commonly used design to study etiologic risk factors

Lens Type and Ulcerative Kerititis

IUD use and primary tubal infertility

Physical Activity and MI

Selection of Subjects based on outcome (disease) status

Assignment of Exposure exposure determined by genetics, environment or behavior of subject

Temporal Relationship of Exposure Exposure and onset of disease precedes investigator followup __________________________ t | | | E O I

Measurement of Risk and Association Risk: Cannot be estimated Association: Odds Ratio (OR) as an estimate of the relative risk (RR)

Validity Confounding Bias: may be difficult to get information on potential confounders because of retrospective nature of exposure; matching on potential confounders will not eliminate confounding

Validity Information Bias: may reduce possibility by "blinding" investigator to outcome when determining exposure status; objective criteria for identifying exposure may also reduce possibility of bias

Validity Selection Bias: as exposure and outcome have occurred prior to selection, the potential exists to use knowledge of exposure status in selecting subjects

Advantages best method for studying rare outcomes normally less expensive than prospective study can be used to study outcomes with long latent periods; quickly done

Limitations must use retrospective information on exposure and confounders cannot estimate incidence problems of validity shared with those of retrospective studies

GUIDE FOR EVALUATING A CASE-CONTROL STUDY Are cases and controls clearly defined? Were these criteria fulfilled in this study? Were cases and controls comparable? –Do cases and controls come from the same population? Did cases and controls enter the study similarly? Did authors adjust for any differences?

GUIDE FOR EVALUATING A CASE-CONTROL STUDY Was exposure status clearly defined and consistently obtained? What was the response rate for cases? controls? Were cases and controls blinded as to the study hypothesis? Was the study size sufficiently large to justify the author's conclusions?

Lecture 7 Objective 19. Describe the elements of design of observational studies: cross ‑ sectional studies. Discuss the advantages and disadvantages of this design.

Crosssectional (Prevalence) Studies Relate the disease outcome and exposure status at the same time. These studies are important in establishing the current health status of the population in diseases of short duration.

Cholera and Water Source

Cough and boys smoking

Selection of Subjects from total population

Assignment of Exposure exposure determined by genetics, environment or behavior of subject

Temporal Relationship Investigator (I), Exposure (E) and Outcome (O) are coincident E |O _____________________ t | I

Measurement of Association Risk: Prevalence (P) Association: Prevalence Ratio (PR) or Prevalence Difference (PD)

Validity Confounding Bias: may be difficult to get information on potential confounder because of retrospective nature of exposure

Validity Information Bias: eliminate through "blinding", although it may be difficult as exposure and outcome are determined simultaneously; reduce possibility with objective diagnostic criteria

Validity Selection Bias: because outcome has occurred prior to selection, potential exists for knowledge of outcome status to influence selection of subjects

Advantages done quickly; no need for follow ‑ up can be used to study diseases with long latent periods good for hypothesis generation

Limitations uses prevalence to measure risk not useful for studying rare diseases not useful for studying rare exposures must use retrospective information on exposure and confounders problems of validity shared with those of retrospective studies