Epidemiology (VIII) Second Half of Class

Question 1

Why do we prefer to use incident cases in case-control studies? What are the cons?

Answer 1

Pros:
- More representative of all persons w/disease
- Exposure is more likely to have preceded the disease
- Recall may be better
Cons:
-Gotta wait for incident cases to occur
- May still miss people who die before diagnoses

Question 2

True or False: In a case-control study, the prevalence of the disease reflects the prevalence of the disease in the population.

Answer 2

False.

We select as many cases as possible, then an arbitrary # of controls to match. However, the prevalence of EXPOSURE in the D- and D+reflects the actual levels in the reference pop

Question 3

Two rules about selecting controls:

Answer 3

1) They must come from the same reference population as the cases
2) Their exposure prevalence should be representative of the source population –> increase sample size!

Question 4

Group matching vs Individual matching in case-control studies

Answer 4

Group matching: frq matching or stratification; select controls so that it matches the distribution of the factor in case

Individual matching: ‘matched pairs’

Question 5

What study should you use if you want to…

• Study common diseases or common exposures
• Study many exposures & outcomes

Answer 5

General Cohort Study

Question 6

What study should you use if you want to…

• Study rare exposures
• Study many outcomes

Answer 6

Specific Cohort Study

Question 7

What study should you use if you want to…

• Study rare diseases
• Study many exposures

Answer 7

Case-control study

Question 8

What study should you use if you want to…

• Study disease or exposure burden
• Don’t have time to do follow-up

Answer 8

Cross-Sectional

Question 9

What study should you use if you want to…

• Identify correlations and generate hypotheses
• Don’t need individual-level exposure measurement

Answer 9

Ecologic Study

Question 10

What measure of disease frequency is a direct measure of risk?

Answer 10

Cumulative Incidence is a direct measure; whereas, incidence rate is only an estimate.

Question 11

When does OR provide a good approximation of relative risk?

Answer 11

When the cases’ and controls’ exposure prevalence is representative of the source populations’ exposure prevalence.

When the disease studied is relatively rare.

Question 12

Ways to interpret an OR

Answer 12

“The odds of cases having been exposed is _x the odds of controls having been exposed”
“The odds of developing disease among people who were exposed is _x the odds of developing disease among those who were not exposed”
“The risk of disease is _x greater among the exposed compared to the unexposed.”

Question 13

AR Total population takes into consideration both

Answer 13

Excess incidence associated with the exposure (additional risk above background)

Prevalence of exposure-How common is the exposure in the population

Question 14

Case control vs Cohort - what’s the difference?

Answer 14

Cohort studies look at people’s exposures and then follows them over time (exposure -> outcome).

Case control studies look back at diseased and nondiseased people’s previous exposures to identify risk factors for that disease. (outcome -> exposure)

Question 15

Selection Bias- What is it and what is the result o fit?

Answer 15

Systematic error in the selection/follow up of subjects. Thus, subjects are not representative of the source population with respect to exposure or outcomes. RR/OR among the study participants differs from that in the source population.

Ex) Nonexposed controls are more likely to participate than exposed controls.

Ex) Sampling from an area with higher lvls of exposure

Question 16

Information Bias (Misclassification)

Answer 16

Systematic error in the measurement of exposures and/or outcomes

Ex) Subjects tend to underreport their drug use

Question 17

Improper selection of subjects or nonparticipation are causes of selection bias in what types of studies?

Answer 17

Case control or cohort.

Selection bias is more likely to occur in case-control and retrospective cohort studies because both exposure & outcome have already occurred by the time of subject selection.

Question 18

Losses to follow up (must be differential) causes selection bias in what types of studies?

Answer 18

Cohort or RCT

Question 19

3 ways to prevent selection bias

Answer 19

• Representative sampling
• Maximize participation
• Minimize loss to follow up

Question 20

Healthy worker effect

Answer 20

A form of selection bias.

People who work, esp in labor-intensive occupations, are healthier than the general population –> results in underestimates of harmful exposures.

Question 21

Information bias- What is it and what are the types?

Answer 21

Collection of inaccurate information on study factors (exposure, disease, confounders).

Can be introduced by interviewers, participants, and instruments.

Recall bias, interviewer bias, differential and nondifferential misclassification.

Question 22

Nondifferential misclassification

Answer 22

Errors in exposure or outcome status occur with approximately equal frequency in groups being compared.

Includes…

• Equally inaccurate memory of exposures in cases/controls.
• Recording and coding errors in records and databases, but equally in cases & controls.
• Using surrogate measures of
• Recall bias is equal in both cases and controls

Question 23

Differential misclassification

Answer 23

Errors in exposure and outcome occur with UNEQUAL frequency in the groups being compared (cases/ctrls; exp/nonexp)

Includes differential recall (e.g. rumination or social desirability bias); case records being collected differently from ctrl records; etc.

Question 24

How to avoid information bias

Answer 24

• Masking interviewers and subjects to the hypothesis
• Using a control group composed of diseased people
• Careful designing questionnaire
• Not using an interview
• Using multiple measurements, the best info sources, and sensitive & specific criteria to define exposure and disease (prevents misclassification)

Question 25

Nondifferential selection bias in a case control study

Answer 25

Participation is related to disease, but not exposure.

Unbiased.

Ex) Way more cases participate than controls, or vice versa.

Question 26

Differential selection bias in a case-control study

Answer 26

Participation is related to both disease and exposure.

Biased.

Ex) Way more cases and nonexposed people participate.

Question 27

Nondifferential selection bias in a cohort study

Answer 27

Losses to follow-up are related to disease, but not exposure. –> RR is unbiased.

Ex) Only cases, but no controls get lost to follow up.

Question 28

Differential selection bias in a cohort study

Answer 28

Losses to follow up is related to both disease and exposure. –> RR is biased.

Ex) Exposed and cases are more likely to be lost to follow up.

Question 29

Information bias in a case control or cohort study has what effect on the OR? (hint: depends on differential or nondifferential)

Answer 29

Nondifferential exposure misclassification: the error is related to disease, but not exposure –> OR/RR biased toward the null.

Differential exposure misclassification: error is related to disease and exposure –> OR/RR biased toward or away.

Question 30

Bias towards the null means

Answer 30

The observed value is closer to H0: RR=1 than the true value.

True RR is above 1 & observed RR is below it.
or
True RR is less than 1 & observed RR above it.

Question 31

Bias away from the null means

Answer 31

The observed value is farther from H0: RR=1 than the true value.

Question 32

Requirements for an exposure to act as a confounder

Answer 32

1) The exposure (e.g. age) must be related to the exposure (e.g. country of residence) under study.
Ex) People from this country are older; Drinkers are more likely to smoke

2) The exposure must also be associated with the outcome.
Ex) Smoking is a risk factor for lung cancer. PTSD is a risk factor for suicide.

3) The confounder can’t be on the causal pathway between the exposure & disease.
Ex) Drinking doesn’t CAUSE smoking; whereas, hand-to-hand combat CAUSES PTSD.

Question 33

Detecting Effect Modification (Interaction)

Answer 33

Homogeneity of effect: Are the stratum-specific measures association similar to each other?

Combined vs independent effects: Is the combined effect similar to what would be expected based on the independent contributions of each exposure alone?

If either of these are ‘no’, then there may be interaction.

Question 34

Additive vs Multiplicative interaciton

Answer 34

Additive interaction: the difference in ABSOLUTE measures of risk between exposed & non exposed differs across strata formed by a 3rd variable.

• The combined incidence is higher/lower than the independent effects of each of the two exposures.
• compare risk differences

Multiplicative interaction: the RELATIVE risk between exposed & nonexposed differs across strata formed by a third variable.

• The combined relative risk is higher/lower than the independent effects of the two exposures.
• compare relative risk (risk ratio, relative ratio, odds ratio)

Question 35

How to deal with confounding

Answer 35

During the design phase:

• Randomization
• Matching (removes possibility of confounding by that variable, but it can be inefficient, youcan’t study that variable anymore, you may accidentally match on other variables too)
• Restriction (exclude any confounding variables, but also limits generalizability)

During the analysis phase:

• Stratify the OR or RRs
• Direct adjustment: remove the effect of the confounding variable (Regression analysis)

Question 36

Confounding variables are true associations- they’re just not causal. They are risk factors, so they can be used to

Answer 36

Identify an at-risk population

Question 37

Effect modification

Answer 37

The operation of one or more factors to produce an effect; the incidence rate of disease in the presence of multiple risk factors differs from the incidence rate expected based on their individual effects.

Question 38

Interaction

Answer 38

The magnitude of the association between the exposure and disease depends on a third factor; the AR or RR/OR is different across strata of a third variable!