case-control studies

Question 1

case-control study

Answer 1

• observation studies allowing researcher to be a passive observer of natural events occurring in individuals with the DISEASE/CONDITION OF INTEREST (CASES) who are compared with people who DO NOT HAVE THE CONDITION OF INTEREST (CONTROLS)
• group-assignments based on DISEASE STATUS
• useful when study a rare disease or investigating an outbreak
• subject land in study groups based on self-selection by natural occurrence (have the disease or not)
• commonly generates an Odds of exposure for each, then and Odds Ratio as the measure of association

-already know disease column totals (A+C and B+D)

Question 2

case-control study flowchart

Answer 2

population

• disease/outcome (Cases) -disease/outcome (controls)
• exposed vs non-exposed -exposed vs non-exposed

Question 3

reasons to select a case-control design

Answer 3

• unable to force group allocation (randomize) due to ethics
• limited resources (time/money/subjects)
• the disease of interest is rare and little is known about its associations/causes
• prospective exposure data is difficult/expensive to obtain and/or very time inappropriate
• usually done in retrospective fashion

Question 4

strengths of case-control studies

Answer 4

• good for assessing MULTIPLE EXPOSURES OF ONE OUTCOME
• useful when diseases are rare
• useful in determining associations (not causation) due to confounders, recall bias, etc.
• less expensive than interventional trials and prospective cohort studies
• useful when ethical issues limit interventional studies
• useful when disease has a long induction/latent period

Question 5

weaknesses of case-control studies

Answer 5

• can’t demonstrate causation
• can be impacted by unassessed confounders
• retrospective: can’t control for other exposures or potential changes in amount of study-exposure during study frame
• can be impacted by various biases –> selection and recall biases
• limited by available data (information from records may be missing)

Question 6

selection of cases in a case control study

Answer 6

defined by the investigator using accurate, medically-reliable, efficient data sources; applied to all study participants objectively and consistently

case definition

misclassification (differential/non-differential) is possible

Question 7

case-control: control group

Answer 7

as close to “case” group minus the disease/outcome

exchangeability = comparability with respect to all other determinants of outcome

controls must be selected irrespective of exposure status

Question 8

most difficult part of case-control study is

Answer 8

control selection

goal is: to assess for the presence of an association between exposure and known condition of interest by selecting non-disease individuals from the sample population which produced the “cases”

expectation is the controls represent the baseline risk of exposure in the general or reference population

Question 9

where can control groups come from in a case-control study

Answer 9

‘population’: can be randomly selected, not randomized

institutional/organizational/provider/payer: e.g. if cases are centered around blood cancer, controls could come from same cancer department in hospital but could differentiate in type of cancer

spouse/relatives/friends: genetic, environmental, soci-economic similarities

Question 10

outbreak-sources of controls

Answer 10

participated in same event

Question 11

cases and controls of the following study: association between use of substances that contain caffeine and the risk of crash in long distance commercial vehicle drivers

Answer 11

cases: long-distance truck drivers who were recently involved in a crash regardless of caffeination
controls: long-distance truck drivers who had not been in a crash regardless of caffeination

Question 12

how can one individual function as both an exposed individual and an unexposed individual in the same study

Answer 12

individual can be associated with an outbreak investigation with multiple exposures. e.g.: studies on exposure to different food at same gathering

Question 13

in a case-control study, if an individual functions as both an exposed individual and an unexposed individual in the same study, its called what?

Answer 13

case-crossover

only case-control design that attempts to address Temporality

Question 14

nested case-control studies

Answer 14

case-control studies conducted after, or out of, a prospective previous study-type (cohort or interventional study)

-subjects in cohort study, ultimately developing disease/outcome, are defined as cases for the subsequent case-control study

Question 15

selection of controls used for Nested Case-Control studies

Answer 15

survivor sampling: sample of non-diseased individuals (survivors) at end of study period

base sampling: sample of non-diseased individuals at start of study period

risk-set sampling: sample of non-diseased individuals during study period at same time when case was diagnosed; when building case-control, your controls are at the same level of risk that the cases were

Question 16

common biases in case-control

Answer 16

selection bias: way subjects are chosen for the study (usually more concerning for control selection; less concerning during case-crossover)

recall bias: related to amount/specificity that cases or controls recall past events differently (more likely that cases are more likely to recall past exposures and levels of exposure)

Question 17

matching

Answer 17

making sure whatever type of individual, or numerically the number of people in each of our observational groups are equal; can match in a 1:1 ratio or higher

• trying to build in exchangeability concept
• higher ratios used to average variability and prevent outliers

Question 18

individual matching

Answer 18

matches individuals based on specific patient-based characteristics

useful for controlling confounding characteristics

Question 19

group matching

Answer 19

requires cases be selected first

proportion of cases and proportion of controls with identical characteristics are matched

e.g. 41 percent of cases are male, so 41 percent of controls are male