As an example, of the 91,52350 womenmen in the [[Nurses' Health Study]]a whobasketball didcoach notuses havea cancernew attraining baselineprogram andto whosee wereif followedit forincreases 14the years,number 2,341of womenplayers hadwho developedare breastable cancerto bypass 1993.a Severalcertain studiesskills havetest, used standard cohort analysescompared to studyan precursorsold totraining breastprogram cancer,The e.g.coach userecruits of50 hormonalplayers contraceptives,<ref name="pmid9051324">{{cite journal|author1=Hankinson SE |author2=Colditz GA |author3=Manson JE |author4=Willett WC |author5=Hunter DJ |author6=Stampfer MJ |display-authors=etal | title=A prospective study of oral contraceptiveto use and risk of breast cancer (Nurses' Health Study, Unitedeach States)program. |34 journal=Cancermen Causespassed Controlthe |new year=program 1997by | volume= 8 | issue= 1 | pages= 65–72 | pmid=9051324 | doi= 101993.1023/a:1018435205695|s2cid=24873830Based |on url=https://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=9051324the old }}</ref>program whichis11 a covariate easily measured on all offailed the women in theskills cohorttest. 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 [[genethe expression]]New program and breast cancerskills 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.
