Analytical Studies Flashcards
What type of design as a case control study?
Retrospective
Why is case control study retrospective?
Both exposure and disease have occurred before onset of study
Which study is often used first to study a suspected association of causality?
Case control
When is a case control study most useful?
In diseases where exposure is common but disease only occurs in small proportion of those exposed
Advantages of case control study
Easy to carry out
Less time consuming
Less expensive
Suitable for investigating rare diseases
Subjects not exposed to new risks
Several etiological factors for single disease can be studies
No attrition problems
Disadvantages of case control studies
Prone to selection and recall bias
Control group selection may be difficult
Incidence cannot be measured
Cannot prove causality
Temporality is difficult to determine retrospectively
Defining a case is crucial - strict diagnostic criteria & eligibility criteria needed
What cannot be calculated in case control studies?
Incidence
Relative risk
Where can cases in case control studies be obtained from?
Hospital or general population
What must controls by definition in case control studies be?
Identical to cases except for presence or absence of disease and recruited independently of exposure status
What can controls be in case control studies?
Hospital patients with different health problem to that studied
Relatives of patients studied
Neighbourhood controls
General population controls
Up to what ratio does the pwer of study increase as number of control increases?
4:1
What is done to ensure comparability of cases to controls in case control studies?
Matched e.g. by age, sex
How can matching occur in case control studies?
Groupwise
Pairwise - more time consuming
What can overzealous matching of controls in case controls lead to?
Difficulty in recruiting controls
Reduce risk difference between two groups
May inadvertently result in matching for putative causal factors
What is used to estimate risk of exposure to case control studies?
Cross-product ratio called odds ratio
Why can incidence rates not be calculated in case control studies?
No longitudinal observation see new cases developing
At what point will odds ratio approximate the relative risk in case control studies?
If disease is very rare - <5%
Values of odds ratio
0 to infinity i.e. no negative values
Implication of there being no negative value for odds ratio?
Log transformations needed to calculate confidence intervals
Describe a cohort study
Group of people sharing common aspect
What is exposure cohort?
Group of people exposed to a risk factor
What is an inception cohort?
Group of patients assembled at a single point of time based on a common factor
What happens in cohort studies?
Exposure cohorts are followed up in parallel in non-exposed group to detect development of disease
When is cohort studies useful?
When exposure is rare but likelihood of disease is notably high after exposure
When are cohort studies only possible?
Easily obtainable, cooperative and stable cohort that can be followed up is available
Criteria for non-exposed control in cohort studies
Comparable to study cohort in all respects except for exposure
Must not have disease at time of inception of study
How can control group in cohort studies be divided?
Internal control
Exetrnal control
What is internal control in cohort studies?
Sbbgroups in exposure cohort according to dose/degree of exposure
How can relative risk be calculated in cohort studies?
Ratio of disease (outcome) in exposed group to disease in non-exposed group
Determination time in cross-sectional studies
Current estimate of exposure and outcome
How are subjects in case-control identified?
According to disease (outcome) status
How are subjects in prospective studies identified?
Exposure status
How are subjects in retrospective studies identified?
Exposure status
Determination time in prospective studies
Current estimate of exposure to future estimate of outcome
Determination time in retrospective stdies
Past estimate of exposure to current or past estimate of outcome
Advantages of cohort studies
Less prone to selection and recall bias
Incidence & relative risk can be measured
Causal associations can be strongly supported
Temporality easily established
Multiple effects of single exposure can be observed
Dose-response relationships can be calculated
Natural course of exposure to disease pathway can be studied
Disadvantages of cohort studies
Time & resource consuming
Not suitable for investigating rare diseases
Only one aetiological factor can be studied at a time
Attrition rate is an issue
What is a nested case-control study?
In which both cases and controls are drawn from within a cohort rather than including entire cohort population in a study
Advantages of nested case-control studies?
Cheaper and can produce same findings with almost same level of precision
