Lexis diagrams (cont.)

Translating data from period to cohort classification

If we have a series of age-specific data in consecutive years, we can translate this into cohort data.

Below is a table which shows the period age-specific fertility rates for Australian women, from 1920-24 to 1950-54.

If we imagined the vertical columns were a cohort (showing age-specific rates), we would be discussing a synthetic cohort. However, as there is a time series of data, from this table we can also construct the experience of an actual cohort.

Women who were 15-19 during the years 1920-24 will be aged 20-24 at some time in the period 1925-29, and will be 25-29 at some time between 1930 and 1934. We can follow the sequence of fertility rates which most of them will experience by looking down the main diagonal of the first panel in the table, starting at the top left-hand corner.

Cohort data Period data Clear

 

Adapted from Pollard (1974)

Age Group 1920 - 24 1925 - 29 1930 - 34 1935 - 39 1940 - 44 1945 - 49 1950 - 54
15 - 19 0.026 0.029 0.026 0.025 0.024 0.030 0.039
20- 24 0.134 0.127 0.106 0.106 0.123 0.156 0.186
25 - 29 0.168 0.156 0.128 0.129 0.147 0.179 0.190
30- 34 0.138 0.123 0.100 0.097 0.109 0.126 0.124
35 - 39 0.098 0.086 0.065 0.058 0.062 0.074 0.066
40 - 44 0.041 0.035 0.027 0.021 0.020 0.023 0.021
45- 49 0.004 0.004 0.003 0.002 0.002 0.002 0.002
Age Group 1920 - 24 1925 - 29 1930 - 34 1935 - 39 1940 - 44 1945 - 49 1950 - 54
15 - 19 0.026 0.029 0.026 0.025 0.024 0.030 0.039
20- 24 0.134 0.127 0.106 0.106 0.123 0.156 0.186
25 - 29 0.168 0.156 0.128 0.129 0.147 0.179 0.190
30- 34 0.138 0.123 0.100 0.097 0.109 0.126 0.124
35 - 39 0.098 0.086 0.065 0.058 0.062 0.074 0.066
40 - 44 0.041 0.035 0.027 0.021 0.020 0.023 0.021
45- 49 0.004 0.004 0.003 0.002 0.002 0.002 0.002

The data highlighted in the diagonal shows how we can translate the period data of the columns into cohort data, by following women through time as they age and experience different rates.

Age Group 1920 - 24 1925 - 29 1930 - 34 1935 - 39 1940 - 44 1945 - 49 1950 - 54
15 - 19 0.026 0.029 0.026 0.025 0.024 0.030 0.039
20- 24 0.134 0.127 0.106 0.106 0.123 0.156 0.186
25 - 29 0.168 0.156 0.128 0.129 0.147 0.179 0.190
30- 34 0.138 0.123 0.100 0.097 0.109 0.126 0.124
35 - 39 0.098 0.086 0.065 0.058 0.062 0.074 0.066
40 - 44 0.041 0.035 0.027 0.021 0.020 0.023 0.021
45- 49 0.004 0.004 0.003 0.002 0.002 0.002 0.002

This column shows age-specific data for the period 1920-24. From this we could construct a synthetic cohort.