A set of training materials for professionals working in intervention epidemiology, public health microbiology and infection control and hospital hygiene.
You can't make decissions on this page's approval status because you have not the owner or an admin on this page's Group.
Jean Claude Desenclos
Case control studies
In cohort studies the denominator represents the exposure experience of the source population. If it is the exposure status as observed at the beginning of the cohort we will compute a risk. If we allow for exposure to vary overtime we will compute a rate which takes into account the time spent by each individual in the exposed and unexposed cohorts over time.
The main constraint in cohort studies is the necessity to collect information on exposure from large populations (to have denominators for the exposed and unexposed cohorts). We will see below that instead of collecting exposure information from the entire study population we can use a sample of it to calculate or estimate the risk ratio or the rate ratio. In other words, instead of using the entire cohort denominator we will use a sample of it. This sample is also frequently called a control group and it is used to represent the exposure experience of the source population.
The rationale behind using a sample of the denominator comes from the following formula for risk and rate ratio which can alternatively be expressed as follows:
From the above formula we already see that if we take an unbiased random sample of Ne and Nu the ratio of exposed to unexposed (Ne/Nu) will not be modified and therefore the risk ratio will remain the same (Ne/Nu or a sample of it gives the same risk ratio if sampling is unbiased). The same applies if we use person years at risk (PT).
We have generally speaking three major ways to select a sampled control group which reflect three ways to estimate exposure experience in the source population.
Join the discussion about this article in the forum!
ecdc posted on 10/17/2012 3:43:34 PM:
OJE published "Case-control studies: basic concepts"
Abs: The purpose of this article is to present in elementary mathematical and statistical terms a simple way to quickly and effectively teach and understand case-control studies, as they are commonly done in dynamic populations-without using the rare disease assumption. Our focus is on case-control studies of disease incidence (‘incident case-control studies'); we will not consider the situation of case-control studies of prevalent disease, which are published much less frequently.
Ref: Jan P Vandenbroucke and Neil Pearce: Case-control studies: basic concepts Int. J. Epidemiol. (2012) 41(5): 1480-1489 doi:10.1093/ije/dys147 http://ije.oxfordjournals.org/content/41/5/1480.full.pdf+html
Arnold Bosman replied on 10/21/2012 1:22:41 PM:
Good reading !
The need to use case control in prevalent diseases occurs often. In that case we need to understand that 'the rare disease assumption' does not really exist, yet we need to think carefully about where we select our controls. The core principle is that we select controls because we want to measure 'how the exposure of interest is distributed in the population that gave rise to the cases.
On this topic, a very readable article appeared in Eurosurveillance. Olivier le Polain de Waroux and his co-authors do a great job describing and explaining the principles of case-cohort, with very useful examples.
I can invite everyone to read this piece:
Le Polain de Waroux O, Maguire H, Moren A. The case-cohort design in outbreak investigations. Euro Surveill. 2012;17(25):pii=20202. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20202
sbpmebxu replied on 7/29/2015 7:35:38 PM: 1
sbpmebxu replied on 7/29/2015 8:10:37 PM: 1
You need to be logged in to post comments.
You can log in here. You can register here if you haven't done so yet.