How are life tables derived? (cont.)
Demographic notation
The columns mentioned so far have headings of q, p and l. We have mentioned l as the most important, l for living, survivors at various ages.
In demography q is always used to denote the risk of dying: p is always used to denote the risk of surviving. Since one can only be alive or dead p is always 1-q; it is the complement of q, and vice versa. Check this for yourself in Table 1.
Occasionally capital Q or P is used - they have the same meaning. Lower case is more conventional.
Note also that q and p have subscripts both before and after the central letter (a prefix and a suffix), but l has a suffix only. All the suffixes are "x" meaning that the number in the column refers to a specific age x. The "n" prefix is the width of the interval.
The fact that some column heads have a prefix and a suffix, while others have a suffix only (see other columns in Table 1) is important - it gives you a good clue about the function of the column.
Figure 4 on the next page explains how these sub-scripts work. Please run through these figures.