The Goldilocks dilemma in survey design and its solution

A dilemma appears in the design of any survey with followups intended to detect the occurrence of an event. Suppose that, at followup, the event of interest can only be determined to have occurred since the previous interview (such is often the case, for example, with onset of dependent behavior in the aged). Then a followup which takes place too soon runs the risk of observing no or very few events, while a followup which is conducted after too long a wait might observe events in nearly every subject, and so be equally uninformative. Thus we would have what one investigator compares to Goldilocks dilemma: is the interval before followup too short, too long, or just right? The problem is multiplied when more than one followup is intended. As a partial solution, a technique is described here which provides guidance on the spacing of followup waves in a multiwave study, preventing possibly serious inefficiencies in study design. Data from the Massachusetts Health Care Panel Study (MHCPS) on functional dependence in the elderly are used for a demonstration.