Lecture 5 (Case-Control Studies)

Question 1

Case control study

Answer 1

-Group of ppl w/disease + comparison grp w/o disease compared w/respect to history of past exposures

Question 2

Diff between case control and cohort study

Answer 2

• Case control: starts w/ppl with and w/o disease, compares history of past exposures
• Cohort: starts w/ppl with and w/o exposure, follows to compare future disease

Question 3

Incidence or prevalence for case control study?

Answer 3

• Incidence preferred
• Prevalence can’t distinguish between risk factors for development vs survival (smtms used so we don’t have to wait for incidence though)

Question 4

Source pop

Answer 4

-Pop that gives rise to cases

Question 5

Case def

Answer 5

-Definite medical criteria to ID cases

Question 6

Case ID

Answer 6

-System for finding all cases who meet criteria and are part of source pop

Question 7

Source pop types

Answer 7

• Pop-based
• Hospital-based
• Nested

Question 8

Pop-based: source pop

Answer 8

-All residents of defined geo area w/o disease X

Question 9

Pop-based: cases

Answer 9

-All new cases of disease X in source pop, over specified period of time

Question 10

Pop-cased: controls and their selection

Answer 10

• Sample (ideally random) of source pop over same period of time
• Selection: random sample f/pop registry, neighbourhood controls, random digit dialling

Question 11

Hosp-based: source pop

Answer 11

• All ppl w/o Disease X who would attend Hosp A if they had it
• Impossible to ID in practice, random sample in general =/= source pop b/c it doesn’t account for referral patterns

Question 12

Hosp-based: cases

Answer 12

-All new cases ID’d in Hosp A over specified period

Question 13

Hop-based: controls

Answer 13

-Sample of patients in Hosp A w/diagnoses other than Disease X over same period (shared risk factors means exposures won’t be representative of general pop)

OR

-Non random sample of pop, most of whom are healthy (but distribution of exposures may not be = between grps)

Question 14

Effective strategies for hosp-based controls

Answer 14

• Limit controls to those hospitalized for disease w/no shared risk factors
• Include variety of diseases in control grp to dilute biasing effects of unknown shared risk factors
• When excluding diseases, only consider diagnosis at current hospitalization

Question 15

Nested: source pop

Answer 15

-Subjects in ongoing concurrent cohort study who did not have Disease X at baseline

Question 16

Nested: cases

Answer 16

-All new cases in cohort over defined follow-up period

Question 17

Nested: controls

Answer 17

-Random sample of cohort who didn’t develop Disease X over same time period

Question 18

Pros of a nested case control study (vs cohort)

Answer 18

• Requires analysis of way less samples/etc. (cohort: all, this: only cases/controls)
• Preservations of specimens for other research

Question 19

Case control: matching

Answer 19

-Selection of controls similar to cases w/respect to factors other than exposure

Question 20

Frequency matching

Answer 20

-Selection such that distribution of matching factors is similar in case/control grps

Question 21

Individual matching

Answer 21

-Each control individually matched to case w/respect to specific factors

Question 22

Implications of matching

Answer 22

• Association between matching factors + disease can’t be studied
• Overmatching: if matching factor is associated w/the exposure of interest –> similar factors between grp
• Must be taken into account throughout analysis

Question 23

Case-control: RR

Answer 23

• Can’t accurately measure incidence b/c we picked #s in both grps, therefore no RR
• Can do OR instead

Question 24

Odds Ratio

Answer 24

• Ratio of odds that cases were exposed vs odds that controls were exposed
• Good approximation of RR in most cases
• Same interpretation as RR

Question 25

Analysis of matched pairs w/dichotomous exposure

Answer 25

++/–: concordant (uninformative)

+-/-+: disocrdant (can produce OR)

Question 26

Pros of case control studies

Answer 26

• Way less time than cohort
• Requires much smaller sample size (if exposure is common, if it’s rare: cohort)
• Much less costly
• Can test hyps
• Useful for studying rare diseases

Question 27

Cons of case control studies

Answer 27

• Past exposures ascertained after the onset of disease (uncertainty for causality, recall bias)
• Prone to selection bias
• Can estimate incidence using OR
• Observational study

Question 28

Reliability

Answer 28

-Reproducibility/replicability of measurement

Question 29

Validity

Answer 29

-Degree to which a measurement measures what it purports to