Biostatistics & Epidemiology

Question 1

Differentiating between case control and retrospective cohort studies

Answer 1

Based on the order in which risks and outcomes are assessed.

Case control studies determine the outcome first and then look for associated risk factors.

Retrospective cohort studies first determine risk factor exposure then determine outcome.

Question 2

Calculating Number Needed to Treat

Answer 2

1/Absolute Risk Reduction

(If absolute reduction is 4%, then 1/0.04 = 25)

Question 3

Negative predictive value

Answer 3

The probability of being free of a disease if the test result is negative

(Unlike sensitivity and specificity, NPV will vary with the pretest probability of a disease). A patient with a high probability of having a disease will have a low NPV, and a patient with a low probability of having a disease will have a high NPV.

Question 4

Hawthorne effect

Answer 4

The tendency of a study population to affect an outcome because these people are aware that they are being studies.

Question 5

Sample distortion bias

Answer 5

Occurs when the estimate of exposure and outcome association is biased because the study sample is not representative of the target population with respect to the joint distribution of exposure and outcome.

Question 6

Information bias

Answer 6

Occurs due to the imperfect assessment of association between the exposure and outcome as a result of errors in the measurement of exposure and outcome status.

Question 7

Confounding bias

Answer 7

Occurs due to the presence of one or more variables associated independently with both the exposure and the outcome.

Question 8

Best study design to determine if there is a difference in incidence of leukemia between two towns.

Answer 8

Cohort study design.

Question 9

Calculating sensitivity, specificity, positive predictive value, negative predictive value (Table)

Answer 9

Question 10

Measure of association in case-control studies

Answer 10

Odds ratio

Exposure odds ratio: Compares the exposure of people with the disease to the exposure of people without the disease

Question 11

Measure of association in cohort studies

Answer 11

Relative risk and relative rate, where people are followed over time for the occurrence of the disease.

Cannot be calculated in case control studies, because the people being studied already have the disease.

Question 12

Common types of systematic errors in statistical studies

Answer 12

Question 13

What problem does a matched study design help control for?

Answer 13

Confounding

Matching variables should always be potential confounders

Question 14

Methods to control confounding

1. Design stage
2. Analysis stage

Answer 14

1. Design stage

Matching

Restriction

Randomization

2. Analysis stage

Stratified analysis

Statistical modeling