POPM 3240 questions with verified
answers already passed
case-control studies - correct answer ✔✔type of observational study design that begins w/the
selection of study subjects on the basis of their disease/outcome status
Subjects are grouped into cases (disease/outcome positive (O+)) and controls (disease/outcome
negative (O-))
Investigators then look backwards in time to assess whether each subject was exposed (E+) or
not (E-) to a given set of exposures of interest
No information about exposure is known when subjects are enrolled in the study
- Once enrolled, we then classify them according to their exposure status
case-control study process - correct answer ✔✔1. Sample individuals from the population of
interest that are O+ and O-
2. Classify each individual on the basis of their E status (exposure positive (E+) and exposure
negative (E-))
3. Compare the proportion of O+ subjects that are exposed and unexposed to the proportion of
O- subjects that are exposed and unexposed
advantages of a case-control study - correct answer ✔✔good for rare diseases
best for investigating source of an outbreak bc the disease has already occurred and most of the
sick people have already been identified
good for identifying multiple risk factors for a disease (useful for disease of an unknown
etiology)
often fast and inexpensive bc they're retrospective
disadvantages of a case-control study - correct answer ✔✔can't calc disease incidence or
prevalence, therefore can't calc relative risk
, can examine only one outcome
most prone to bias bc E and O have already occurred
temporal relationships are an issue, can be troublesome when trying to point to risk factors that
are attributed to someone developing outcome of interest bc might be related to
characteristics/factors attributed to subjects having/living w/outcome of interest
control selection may be difficult
case definition - correct answer ✔✔clear and explicit set of criteria that minimize the likelihood
of a true case being missed (criteria are sensitive) and of a non-affected person being falsely
classified as a case (criteria are specific)
ex. Symptomatic description (vomiting, diarrhea, etc.), lab confirmation (culture, serology), time
frame (date of diagnosis), and geographic location (Wellington County)
study base - correct answer ✔✔population from which cases and control are obtained
primary study base - correct answer ✔✔population from which cases that arise can be easily
defined ex. kids at a daycare, livestock on a farm
we have a good sense of the population all of the cases are coming from
secondary study base - correct answer ✔✔population of potential study subjects that are one
or more steps removed from primary population ex. patients at a referral clinic, hospital, cancer
registry, or lab
Ex. if we sampled dogs being treated at OVC, we might have a difficult time identifying where
these dogs came from bc OVC is a specialist referral centre, so they probably came from
Ontario, but it's hard to be sure
bias in case-control studies - correct answer ✔✔selection bias - control group should reflect
prevalence of exposure in non-diseased members of population from which cases were drawn
information bias - recall bias (problems recalling the past), partic if accuracy differs b/w cases
and controls
answers already passed
case-control studies - correct answer ✔✔type of observational study design that begins w/the
selection of study subjects on the basis of their disease/outcome status
Subjects are grouped into cases (disease/outcome positive (O+)) and controls (disease/outcome
negative (O-))
Investigators then look backwards in time to assess whether each subject was exposed (E+) or
not (E-) to a given set of exposures of interest
No information about exposure is known when subjects are enrolled in the study
- Once enrolled, we then classify them according to their exposure status
case-control study process - correct answer ✔✔1. Sample individuals from the population of
interest that are O+ and O-
2. Classify each individual on the basis of their E status (exposure positive (E+) and exposure
negative (E-))
3. Compare the proportion of O+ subjects that are exposed and unexposed to the proportion of
O- subjects that are exposed and unexposed
advantages of a case-control study - correct answer ✔✔good for rare diseases
best for investigating source of an outbreak bc the disease has already occurred and most of the
sick people have already been identified
good for identifying multiple risk factors for a disease (useful for disease of an unknown
etiology)
often fast and inexpensive bc they're retrospective
disadvantages of a case-control study - correct answer ✔✔can't calc disease incidence or
prevalence, therefore can't calc relative risk
, can examine only one outcome
most prone to bias bc E and O have already occurred
temporal relationships are an issue, can be troublesome when trying to point to risk factors that
are attributed to someone developing outcome of interest bc might be related to
characteristics/factors attributed to subjects having/living w/outcome of interest
control selection may be difficult
case definition - correct answer ✔✔clear and explicit set of criteria that minimize the likelihood
of a true case being missed (criteria are sensitive) and of a non-affected person being falsely
classified as a case (criteria are specific)
ex. Symptomatic description (vomiting, diarrhea, etc.), lab confirmation (culture, serology), time
frame (date of diagnosis), and geographic location (Wellington County)
study base - correct answer ✔✔population from which cases and control are obtained
primary study base - correct answer ✔✔population from which cases that arise can be easily
defined ex. kids at a daycare, livestock on a farm
we have a good sense of the population all of the cases are coming from
secondary study base - correct answer ✔✔population of potential study subjects that are one
or more steps removed from primary population ex. patients at a referral clinic, hospital, cancer
registry, or lab
Ex. if we sampled dogs being treated at OVC, we might have a difficult time identifying where
these dogs came from bc OVC is a specialist referral centre, so they probably came from
Ontario, but it's hard to be sure
bias in case-control studies - correct answer ✔✔selection bias - control group should reflect
prevalence of exposure in non-diseased members of population from which cases were drawn
information bias - recall bias (problems recalling the past), partic if accuracy differs b/w cases
and controls
