A number of sampling techniques are available:

• Survivor Sampling: recruit all of the controls following the accrual of all the cases.
• Base Sampling: recruit all of the controls at the beginning of the study, and then allow cases to accrue.
• Risk Set Sampling or Incidence Density Sampling: Recruit controls at the time each case is ascertained.

Each method has its pros and cons. Some are logistically more difficult and expensive but will allow you to better estimate the rate ratio or the risk ratio. Others are simpler and more efficient but require important assumptions to estimate particular measures of association. After weighing all of the many factors, you decide to employ a method that epidemiologists term survivor or cumulative incidence sampling in which you select all of your controls after you have already selected all of your cases from the hospitalized patients with Susser Syndrome from the General Hospital (Aschengrau & Seage pp.243-244).