Observational Studies 2

What is a cross-sectional study?

Examines the relationship between diseases and other variables of interest as they exist in a defined population one particular time. Assess relationship between current disease prevalence and current exposure prevalence.

What are strengths of cross-sectional studies?

Quick and easy to conduct. Low cost. Generalizable.

What are limitations of cross-sectional studies?

Single point in time “snapshot” - can only access prevalence. Cannot infer temporality if exposure status may change. Usually prevalent cases of long duration.

What is an ecological study?

Examines rates of disease in relation to a factor described on a population level. Unit of analysis is the population, not an individual. Exposure status is measured at the population level.

What is are advantages of Ecological studies?

Low cost, wide range of exposure levels, ability to examine contextual factors.

What are limitations of ecological studies?

Ecological fallacy, lack of information on confounding variables.

What is a cohort?

A cohort is a group of people with a common characteristic or experience, often followed forward in time.

What is a prospective cohort study?

Group people based on past or current exposure status and follow into the future.

What is a retrospective cohort study?

Both the exposure and outcomes have already occurred.

What is an open population (dynamic)?

May gain or lose members - can calculate incidence-rate

What is a fixed population?

Does not gain members but may lose them - can calculate incidence rate

What is a closed population?

Does not gain or lose members - can calculate cumulative incidence

What are ways of assessing exposure?

Interview. Questionnaire. Health records. Employment records. Environmental monitors. Biospecimens.

What are ways of assessing outcome?

Health records. Questionnaire. Employment records. Interview. Physical exam/measurements. Biospecimens. Registries.

What is the issue of cohort retention?

Loss to follow-up. Reduces the sample size and the ability of a study to detect an association. Their missing data biases the study (selection bias)

What are considerations for length of cohort studies?

How many years you are willing and financially able to wait. Causal action of an exposure on outcome: occurs when sufficient exposure has accrued.

What is induction and latent period?

Between action of an exposure and disease onset. Interval between disease onset and clinical diagnosis.

What are advantages of prospective cohort studies?

Preserves temporality. Directly measures incidence rates. Minimize recall and reverse causality bias. Exposures and potential confounders can be accurately and precisely measured by trained staff at the start. Can investigate multiple exposures and outcomes.

What are limitations of prospective cohort studies?

Biases (selection bias, confounding). Resource intensive.

What are advantages of retrospective cohort studies?

Time effective, cost effective, can directly assess risk, good for rare exposures, good for long-term outcomes.

What are limitations of retrospective cohort studies?

Rely upon databases - no control over how exposure, outcome, confounders were measured (misclassification, incomplete data, poor quality)