Case-Control Studies

Question 1

What is the hierarchy of Epidemiologic Study Designs (7)?

Answer 1

Case Reports (useless), Case Series, Ecologic Studies, Cross-Sectional Studies, Case-Control Studies, Cohort Studies, Randomized Controlled Trials

Question 2

What are some of the differences between Case-Control and Retrospective/Prospective Cohort studies?

Answer 2

Case-control studies: you can’t use the RR, you can only use the OR, good for rare diseases; figure out what caused the disease
Prospective Cohort - monitor exposure and the onset of disease, conduct interviews, level of exposure is more understood, RR and OR are both relevant for this. Can test a new treatment
Retrospective Cohort - RR and OR are both relevant, looking backward

Question 3

Definition of Case-Control Study

Answer 3

The case-control study in epidemiology is an analytic epidemiologic research design which the study population consists of groups who either have (cases) or do not have a particular problem or outcome (controls).

Question 4

Where does the investigator look in a Case-Control study and what do they measure?

Answer 4

They look back in time to measure the exposure of the study subjects. The exposure is then compared among cases and controls to determine if exposure could account for the health condition of the cases.

Question 5

Characteristics of Case-Control Studies (7)

Answer 5

Observational/Non-experimental
Occasionally exploratory
Explanatory (Analytical)
Retrospective (Look backward)
Effect to Cause (What made them sick?)
Both exposure and disease have already occurred
Uses comparison group (Controls)

Question 6

Basic overview of Case-Control design

Answer 6

The investigator selects cases with the disease, and the appropriate controls without the disease and obtains data regarding past exposure to possible etiologic factors in both groups. The investigator then compares the frequency of exposure of the two groups.

Question 7

What are cases made up of?

Answer 7

Those exposed and not exposed WITH the disease

Question 8

What are controls made up of?

Answer 8

Those exposed and not exposed WITHOUT the disease

Question 9

Odds Ratio in Case-Control Studies

Answer 9

In CCS we ask whether certain exposures are more common among those with the disease. Here we are sampling only a portion of potentially exposed people. These differences mean that with CCS we rely on odds ratios to determine whether exposure to a risk factor is more common among cases or controls.

Question 10

How does relative risk help a CCS?

Answer 10

Helps determine whether incident of disease is associated with certain exposures

Question 11

Calculation for Odds

Answer 11

Odds for cases = # cases exposed to RF/ # cases NOT exposed to RF
Odds for controls = # controls exposed / # controls not exposed

Question 12

Sources for controls (6)

Answer 12

Population of defined area
Hospital patients
Probability sample of the total population
Neighbors (walk, hone, letters)
Friends of cases
Siblings, spouses, relatives
Other

Question 13

Odds Ratio Calculation

Answer 13

Odds cases/Odds Controls

Question 14

Odds Ratio Explanation

Answer 14

You had a ___ times greater chance of becoming ill if ____ as compared to those that did not

Question 15

Calculating confidence interval for OR

Answer 15

1. Calculate error factor e= e1.96x SqRt (1/a + 1/b +1/c +1/d)
2. Lower limit = OR/EF
3. Upper limit = ORxEF
4. Explanation - There is a 95% probability that the true Odds Ratio for the entire population lies between ___ and ____.

Question 16

Bias

Answer 16

Any systematic error in the study design that results in an incorrect estimation of the association between exposure and risk of disease

Question 17

Information bias

Answer 17

Differences in the way exposure or outcome are measured between controls and diseased groups

Question 18

What are observers’ impact on bias and how do researchers mitigate/reduce it (3)?

Answer 18

Observers are privy to the desired outcome and influence data.
Reduce bias by:
1. Blind studies
2. Strict protocols
3. Standardized questionnaires

Question 19

Recall bias

Answer 19

The individuals being questioned have different levels of recall. Cases, due to the trauma of illness, are likely to better recall exposures than controls. (Children, elderly)

Question 20

What does recall bias lead to?

Answer 20

Over or under estimations or associations between exposure and outcome

Question 21

Selection bias

Answer 21

Poor selection of controls, in which there are differences beyond exposure

Question 22

How do you reduce selection bias?

Answer 22

Choose controls with the same demographics and exposure

Question 23

How do you calculate OR from a 2x2?

Answer 23

a x d / b x c = OR

Question 24

Confounding

Answer 24

Confounding is when another variable, outside the one being considered, may influence the outcome of disease (Ex - alcoholics more likely to smoke)

Question 25

Effects of confounding

Answer 25

Can mislead the investigator to make associations or miss associations between a risk factor and disease outcome when none really exists.
Can cause and over or under estimation of a risk factor’s effect

Question 26

Advantages of CCS (4)

Answer 26

1. cheap, easy, quick
2. multiple exposures can be examined
3. rare diseases and diseases with long latency can be studied
4. suitable when randomization is unethical

Question 27

Disadvantages of CCS (8)

Answer 27

1. cases and control selection are troublesome
2. subject to bias
3. direct incidence estimation is impossible
4. temporal relationship is unclear
5. multiple outcomes cannot be studied
6. if the incidence of exposure is high, it is difficult to show the difference between cases and controls
7. not easy to estimate the attributable fraction
8. reverse causation is a problem in interpretation (especially molecular)