the panel to evaluate the assumption that transition rates were stable.^{18}

Since during the course of a career an individual could have more than one transition to a retirement state, each transition was treated as a separate observation in the retirement rate calculations. To calculate the proportion of retiring biomedical or behavioral scientists, average age-specific retirement rates for combined six-year periods of time were used as model parameters. These rates were calculated by first summing the total number of persons retiring in three separate pairs of survey cycles. For example, survey years 1973 and 1975 represent one pair of survey cycles; persons employed in 1973 but retired in 1975 were counted as "retired" in this pair of survey cycles. Survey years 1973-1975, 1975-1977, and 1977-1979 represent three pairs of survey cycles. The total number of persons retiring in three pairs of survey cycles was then divided by the total number of persons in the workforce during the earlier year of each pair of survey cycles (e.g., 1973 in the 1973-1975 pair of survey cycles). In computing these retirement rates, the data were aggregated over both the biomedical and behavioral science workforces to enhance sample sizes. This aggregation assumes that age-specific retirement rates do not differ significantly between these two workforces.

Field mobility was defined as movement out of or into the field of biomedical or behavioral science. As with retirement rates, the mobility rates were based on the observed proportion of personnel moving out of or into a field over a two-year period. In the case of biomedical out-migration, for example, the rate was defined as the proportion of persons moving from the biomedical workforce state to a nonbiomedical workforce state between two survey cycles. The biomedical in-migration rate was defined as the proportion of persons moving from the nonbiomedical workforce to the biomedical workforce over a two-year period. As noted earlier, the base (or denominator) of these proportions was not the same for the two types of mobility. For outflows it was the field-specific workforce; for inflows it was the nonbiomedical or nonbehavioral workforce.

The methods used to calculate the mobility rates by age category for the two-year periods and for combined periods of time were the same as those described for retirement rates. Similarly, the methods used to calculate mobility rates for the behavioral workforce were the same as those described for the biomedical workforce.

The model includes four disjoint workforces: biomedical, nonbiomedical, behavioral, and nonbehavioral.^{19} The number flowing into the biomedical (or behavioral) workforce is constrained by the model to be equal to the number flowing out