Nested case–control study: Difference between revisions

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==Example==

As an example, of the 91,523 women in the [[Nurses' Health Study]] who did not have cancer at baseline and who were followed for 14 years, 2,341 women had developed breast cancer by 1993. Several studies have used standard cohort analyses to study precursors to breast cancer, e.g. use of hormonal contraceptives,<ref name="pmid9051324">{{cite journal|author1=Hankinson SE |author2=Colditz GA |author3=Manson JE |author4=Willett WC |author5=Hunter DJ |author6=Stampfer MJ et al.|display-authors=etal | title=A prospective study of oral contraceptive use and risk of breast cancer (Nurses' Health Study, United States). | journal=Cancer Causes Control | year= 1997 | volume= 8 | issue= 1 | pages= 65–72 | pmid=9051324 | doi= | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=9051324 }}</ref> which is a covariate easily measured on all of the women in the cohort. However, note that in comparison to the cases, there are so many controls that each particular control contributes relatively little information to the analysis.

If, on the other hand, one is interested in the association between [[gene expression]] and breast cancer incidence, it would be very expensive and possibly wasteful of precious blood specimen to assay all 89,000 women without breast cancer. In this situation, one may choose to assay all of the cases, and also, for each case, select a certain number of women to assay from the risk set of participants who have not yet failed (i.e. those who have not developed breast cancer before the particular case in question has developed breast cancer). The risk set is often restricted to those participants who are matched to the case on variables such as age, which reduces the variability of effect estimates.