Cohort Change

Studies of cohort change emphasize a dynamic different from that employed by studies of individual change and development. As an example, consider polls that measure approval ratings for elected officials. To examine change at the individual level, it is necessary to interview and reinterview the same respondents. Each time a respondent registers an approval level, the question of change requires a comparison to that individual's previous responses. Changes can be linked to respondents' experiences during the interval. Respondents could alternate between positive and negative reactions to an elected official as the official's actions in turn pleased and then displeased them. Although this type of volatility in approval ratings may carry certain disadvantages, so long as an equal number of people are switching directions, they cancel each other out. The official could therefore enjoy a majority approval rating on a continuous basis, even though the vast majority of people voiced disapproval sometime during the measurement period. In other words, the balanced magnitude and direction of gross change (change at the individual level), results in no change in the approval rating (zero net change). While the composition of those citing favorable versus unfavorable opinions may change, at any given time the official may continue to do a "good" job according to the majority of constituents. Gross change, therefore, refers to the volume of change at the individual level (how often individual evaluations shift from positive to negative or vice versa), whereas net change refers to a change at the aggregate level (i.e., whether the overall approval rating is higher or lower).

Under conditions of relative stability, these two measures of change may not be very different. For example, if one assumes that retirement is an absorbing state (i.e., a state that, once entered, cannot be vacated), then once a member of a particular cohort shifts from employment to retirement, no subsequent shifts occur. As a consequence, the labor-force participation rate characterizing that cohort progressively declines. However, if retirement (at least within certain age ranges) is a temporary status, then the labor-force participation rate for a given cohort can remain stable, even though individual members of the cohort are regularly moving in and out of employment. In contrast, comparing different cohorts who occupy the same age range at different points in time may reveal the impact of cohort replacement. If the average age of retirement is gradually declining, then comparing different cohorts at ages sixty to sixty-four, for example, will also show a decline in labor-force participation rates, as progressively more members of successive cohorts move into retirement at younger ages, demonstrating that the timing of the retirement transition is changing.