Lecture 17

Question 1

design basics for case-control studies

Answer 1

cases (groups w. outcome) are compared to controls (groups w/o outcome)
- controls provide estimates of frequency off the exposures of interest in source population
- cases and controls are chosen w/o regard to exposure status

Question 2

basic steps in case control study

Answer 2

1. state the research question
2. design the case-control study
3. conduct the case control study
4. analyze and report the data

Question 3

developing a hypothesis for case control studies

Answer 3

• used when investigators are interested in a particular health outcome and want to identify the cause(s) of that outcome
• often used to identify the source of an outbreak or identity risk factors for an outcome that is rare or has a long latency period

Question 4

data layout and measures of association

Answer 4

• case control measured in 2x2
• measures of association: exposure odds ration (EOR)

Question 5

Odds ratio

Answer 5

EOR = AD/BC
- cases: probability of the exposure was among those with the outcome of interest in the source population
- controls: provide comparison; probability of the exposure in the source population
- null value = 1; no association between he exposure and outcome
- interpretation: the odds of exposure among the cases is [insert EOR value] times the odds of exposure among the controls

Question 6

why can’t risk, rate, and prevalence be measures in these studies?

Answer 6

because we’re interesting in the exposure not the outcome

Question 7

ultimate goal of case-control study

Answer 7

determine whether the frequency of the exposure among the cases is more or less than the exposure among controls
- case control studies have a primary and secondary base type

Question 8

primary base

Answer 8

begins with the identification of source population
- ensures cases and controls are samples from same population
- common with outbreaks investigations

Question 9

secondary base

Answer 9

used when investigators lack a well-defined source population
- investigators begin by identifying a mechanism for selection cases from the source population

Question 10

sampling cases after enumeration (listing) of all cases

Answer 10

identify all cases occurring in the source population and include all in study (may not be possible to include all cases)
- random sample: each case has the same probability of being selected

Question 11

sampling cases without enumeration of all cases

Answer 11

occurs when theres no way to identify all individuals with the health outcome
- possible that sampled cases differ from those that would be selected if possible cases could be identified
- - could lead to selection bias

Question 12

incidence vs. prevalence

Answer 12

incident = new
prevalence = existing
- could lead to selection bias

Question 13

identifying controls

Answer 13

should be selected from the same source population if possible

Question 14

population based controls

Answer 14

controls are sampled from a roster that includes all individuals in the source population (birth records; student roster, etc)

Question 15

advantages to population based controls

Answer 15

exposure distribution of the controls should be the same as the source population (given random sampling and high participation

Question 16

disadvantages to population based controls

Answer 16

• may be less motivated to participate in the study
• must be able to enumerate entire source population

Question 17

hospital and clinic based controls

Answer 17

group of individuals who would be treated at the facility where the cases were identified if they were to develop the outcome of interest

Question 18

advantages of hospital & clinic based controls

Answer 18

• no need to enumerate entire source pop
• confounding factors may be similar (education, neighborhood, etc)
• easier and lower cost to identify
• may be more willing to participate

Question 19

disadvantages to hospital & clinic based controls

Answer 19

exposure distribution of the controls may not be the same as the source population

Question 20

relatives, neighbors, friends of cases

Answer 20

if the source population isn’t easily enumerated, individuals who are known to the cases may be sample

Question 21

advantages to relatives, friends of cases

Answer 21

• no need to enumerate the entire source population
• many confounding factors may be similar
• easier and lower cost
• may be more willing to participate

Question 22

disadvantages to relative, friends of cases

Answer 22

exposure distribution of the controls may not be the same as the source population
- cases may not be willing to share contract information with investigator

Question 23

Matching

Answer 23

section of controls with specific attributes that correspond to those of the cases
- only done for factors that are likely to be confounders of the relationship between the exposure and the outcome (age, sex, race)

Question 24

Pair matched

Answer 24

one control is matched to each case

Question 25

n-to-one matching

Answer 25

more than one control to each case

Question 26

category matching

Answer 26

select a control whose variables falls within the same category as the case

Question 27

caliper matching

Answer 27

identification of controls with a value within a specified range of that of the case

Question 28

matching advantages

Answer 28

- can help prevent confounding by the matched factors - can reduce the influence of random error

Question 29

limitations to matching

Answer 29

- makes it difficult to identify controls for inclusion in the study - can be time consuming & costly - once factor has be matched, it can't be considered as a potential exposure in the analysis

Question 30

exposure pattern

Answer 30

determine whether the exposure status of the case and control are concordant (same) or discordant (different)

Question 31

concordant

Answer 31

both the case and control are exposed or both unexposed

Question 32

discordant

Answer 32

either the case is exposed and the control is or vice versa

Question 33

pair-matched odds ratio (moR)

Answer 33

ratio of discordant pairs - >1: more pairs where the case is exposed and the control is not - <1: more pairs where the control is exposed while the case is not

Question 34

interpretation of moR

Answer 34

after controlling for the matched factors, the odds of the exposure among cases are [moR value] times the odds of exposure among controls

Question 35

strength of case-control studies

Answer 35

study of multiple exposures - useful when the cause of outcome is unknown, or there is a need to consider several exposures at once study of rare outcomes - well suited for the study of rare outcomes, as well as outcomes with long induction or latency periods and outcomes that have already occurred logistics - investigators can obtain an estimate of the frequency of the exposure in the source population without actually having to measure the exposure status of everyone in the source population - requires smaller sample size - less resource intensive - can often be conducted faster than RCT or cohort study

Question 36

Limitations of case-control studies

Answer 36

study of rare exposures - investigators need to enroll a very large number of cases and controls to have an adequate number of exposed individuals to be able to make a meaningful comparison limited to a single outcome examining outcome frequency - it is not possible to estimate the frequency of the outcome temporality - there are times when investigators must rely on self-report of exposures after the outcome has occurred -- makes it difficult to establish temporality between the exposure and outcome relative to a controlled trial or cohort study Biased participant selection - if the source population has not been properly identified or if there is bias in the selection of either cases or controls, the results may be incorrect. Selection bias - may occur if investigators select prevalent (vs. incident) cases Biased exposure measurement - exposure information may be susceptible to errors in reporting of the exposure

Question 37

cross sectional study design basics

Answer 37

- exposures / outcomes measured at the same time; provides snapshot - used to asses health status of population & tract changes over time - study participants are not followed - uses descriptive and analytic epidemiology

Question 38

basics steps in cross sectional study

Answer 38

1. state research question 2. design the cross sectional study 3. conduct the cross sectional study 4. analyze and report the data

Question 39

two goals in a cross sectional study

Answer 39

1. estimate the prevalence of health related exposures and/or outcomes in particular population (prevalence) 2. test one or more hypotheses to understand the relationship between exposures and health outcomes (hypothesis testing)

Question 40

prevalence

Answer 40

measure of frequency that describes how common a given characteristic is at one particular point in time

Question 41

prevalence estimation

Answer 41

- understand the scope of the problem - determine whether certain groups of individuals are more likely to be affect - identify potential strategies to improve health outcomes

Question 42

strengths of a cross sectional study

Answer 42

- study of multiple exposures - study of multiple health related outcomes - logistics (less expensive and faster) - avoids loss to follow up - utility for public health practice - aids with future study planning

Question 43

limitations to cross sectional studies

Answer 43

temporality - hard to establish cause and effect study of outcomes or exposures with short duration - has a short duration, PI may obtain an underestimate of exposure or outcome prevalence survival bias - may be ill-equipped to study exposures that tend to either increase or decrease someones likelihood of survival