Study cohort by year of hire and by year of leaving employment
| Year | Hire (n) | Leaving employment (n) |
|---|---|---|
| 1947 | 36 | 0 |
| 1948 | 68 | 0 |
| 1949 | 12 | 16 |
| 1950 | 9 | 30 |
| 1951 | 30 | 10 |
| 1952 | 45 | 6 |
| 1953 | 19 | 14 |
| 1954 | 31 | 22 |
| 1955 | 44 | 12 |
| 1956 | 68 | 13 |
| 1957 | 19 | 20 |
| 1958 | 22 | 12 |
| 1959 | 31 | 21 |
| 1960 | 35 | 22 |
| 1961 | 13 | 19 |
| 1962 | 12 | 16 |
| 1963 | 19 | 15 |
| 1964 | 46 | 24 |
| 1965 | 65 | 26 |
| 1966 | 74 | 28 |
| 1967 | 17 | 69 |
| 1968 | 28 | 28 |
| 1969 | 14 | 31 |
| 1970 | 11 | 17 |
| 1971 | 44 | 20 |
| 1972 | 11 | 34 |
| 1973 | 35 | 26 |
| 1974 | 48 | 26 |
| 1975 | 20 | 18 |
| 1976 | – | 20 |
| 1977 | – | 30 |
| 1978 | – | 13 |
| 1979 | – | 25 |
| 1980 | – | 18 |
| 1981 | – | 19 |
| 1982 | – | 23 |
| 1983 | – | 20 |
| 1984 | – | 5 |
| 1985 | – | 7 |
| 1986–92 | – | 151 |
| Total | 926 | 926 |