Case Control studies

Question 1

what is a case control study

Answer 1

observational study of people with disease of interest and suitable control group of people without disease
= to identify risk factors for an outcome

Question 2

what is the relationship between a potential risk factor and disease examined by comparing the two groups on?

Answer 2

whether the risk factor is present
how frequent the risk factor is
the amount of the risk factor (e.g dose)

Question 3

what is a retrospective study

Answer 3

starts by knowing who has the disease and work back to find who was exposed
so starts after onset of disease and looks back at risk factors

Question 4

what is a prospective retrospective study

Answer 4

cases and controls in case-control study can be accumulated prospectively
as each new case is diagnosed, it is entered into the study BUT still looks back retrospectively from disease to exposure

Question 5

what is the approach of case controls studies

Answer 5

figure out if diseased cases differ from non-diseases controls in terms of prior exposure
compare frequency of exposure

Question 6

why cant disease incidence rate be calculated for case-control study

Answer 6

because it doesnt follow a disease-free population over time

Question 7

what does an odds ratio of 1 indicate

Answer 7

no association between exposure and disease

Question 8

what does an odds ratio of <1 suggest

Answer 8

potential protective effect

Question 9

what does an odds ratio of >1 indicate

Answer 9

positive association between exposure and disease, meaning exposure may be a risk factor

Question 10

what are the essential features of a case-control study

Answer 10

directionality (outcome to exposure)
timing (retrospective for exposure)
rare or new disease - always the design choice if disease is rare

Question 11

how do you choose cases in case control study

Answer 11

define outcome - standard diagnostic criteria and severity/duration of disease
age range for inclusion
temporal boundaries
geographical boundaries of study

Question 12

how do you source cases for case-control study

Answer 12

population-based - identify and recruit all incident cases from population using disease registry, vital records, or defined area
hospital based - identify cases where you can find them BUT may not be representative

Question 13

what should the control group represent

Answer 13

the population of individuals who would have been identified and included as cases had they developed the disease
all exclusions or restrictions must apply to cases and controls

Question 14

outline population-based controls

Answer 14

usually first choice
drawn from specific geographical area and time
randomly select controls from same area and same time period
use government lists, NHS, electoral role or telephone survey

Question 15

what are advantages of population controls

Answer 15

exclusion criteria is easy to apply
cases and controls are from the same study base
representative of whole study base

Question 16

what are disadvantages of using population controls

Answer 16

low participation rates could cause possible bias
inconvenience of finding controls
recall bias - could have been modifications since diagnosis or changes in past habits
may not be motivated to take part

Question 17

what are neighbourhood controls

Answer 17

selected from residences in same geographical area as cases
attempts to reduce variability of socioeconomic status

Question 18

what are advantages of neighbourhood controls

Answer 18

selection doesnt need roster of people - avoids problem of using telephone lists
possible risk factors which vary georgraphically may be more balanced between cases and controls

Question 19

what are disadvanatges of neighbourhood controls

Answer 19

costly
possibly not representative of study base
increased chance of selection of people living alone
difficult to document non-response
possible over-matching if exposure is related to residence

Question 20

outline hospitalised or disease registry controls

Answer 20

control diseases should be subject to same surveillance and detection procedures as cases
controls come from same source as cases e.g hospital admissions, and disease registries

Question 21

what is an example of a hospitalised control

Answer 21

study of smoking and MI among women
cases from admission to coronary care units at 152 hospitals
controls drawn from admissions to other wards in same hospital

Question 22

what are advantages of hospitalised controls

Answer 22

easy to identify controls, readily available in sufficient numbers
more likely to be aware of previous exposures than healthy individuals
more likely to cooperate than healthy individuals - reducing bias due to non-response

Question 23

what are disadvantages of hospitalised controls

Answer 23

they are ill
disease may have common aetiology or be on the causal pathway
more likely to smoke, more likely to be from lower SES, more likely to be heavy drinkers
-bias
if taken from specialist hospitals there could be differences in SES

Question 24

what are other categories of controls?

Answer 24

friends - cheap, convenient, but could over-match when related to exposure
relatives - useful if genetic factors are involved, but there is risk of over-matching

Question 25

how many control groups should there be

Answer 25

one but can have 2
second is a replication group
but both groups should be independant - no overlap

Question 26

what can be matched for in order to control for confounding factors?

Answer 26

sex
date of birth
georgraphical area
SES
maternal age

Question 27

what are disadvantages of matching

Answer 27

increased effort to find suitable controls
exclusion of cases if no match is found
longer study duration (control selection delayed until case identified)
OVERMATCHING

Question 28

what is overmatching

Answer 28

when a matching variable is involved in or is closely related to the mechanism whereby the exposure influences disease risk
- could be on causal pathway
could be a surrogate
could be strongly associated with BOTH disease and exposure
overmatching will bias the relative risk toward 1

Question 29

what is an example of overmatching

Answer 29

study of smoking and lung cancer - matched on carrying a cigarette lighter or not

Question 30

what are the rules of matching

Answer 30

only match on risk factors whose confounding effects need to be controlled for but are not of scientific interest in the study
a matched study is best to have a matched statistical analysis

Question 31

how many controls should there be per case when matching

Answer 31

1:1 is good
increasing number of controls per case increases statistical power but 1:4 is as far as it should go
inefficient beyond this point

Question 32

what is a nested case-control study

Answer 32

cases and controls drawn from cohort study
useful when wanting more detailed measurements on subset of cohort
also for matching on confounders
otherwise the same as a normal case-control study

Question 33

what is an example of a case control study

Answer 33

serum vitamin D status in group of migraine patients compared with health controls
70 migrainers 70 healthy
matched on age and sex
result: signficant negative association between migraine headache and serum vitamin D
significant trend with decreasing odds with increasing serum vitamin D

Question 34

what care needs to be taken in case-control studies

Answer 34

care in study design, especially with issues such as control selection and matching

Question 35

what are advantages of case-control study

Answer 35

several aetiological factors for single disease
optimal for the study of rare disease
well suited for disease with long latency period
relatively quick
small number of subjects so allows more detail assessment
inexpensive
yields relative risk estimate

Question 36

what are disadvantages of case control study

Answer 36

potential for selection bias - in controls and matching
retrospective collection of information - recall is not always reliable
can only look at ONE disease
yields relative risk not absolute risk