Brian S. Everitt A Handbook of Statistical Analyses using SPSS

10.3 Analysis Using SPSS
10.3.1 Sexual Milestone Times
In the sexual milestone times study, the event of interest is the time of
first sexual intercourse and so the survival times are the ages at first sex.
For some women these times are censored since, at the time of the study,
they had not experienced intercourse; all that is then known is that the
time of first sex is greater than their age at the time of the study.
The format of the Data View spreadsheet for the times to first sexual
intercourse is very similar to that of Table 10.1. The rows of the spreadsheet
refer to women and the columns provide information on diagnostic group
(factor diagnos), the women’s age at first sex (agesex) (possibly censored),
the women’s age at the time of the study (age), and an event identifier
(sexstat). The latter takes the value “1” when the event of interest, here
first sex, has occurred by the time of the study (survival time not censored).
Otherwise the value is “0,” indicating censoring so that age and agesex
will have the same value and we know only that age at first sex is greater
than this value.
For this study, the main interest lies in comparing ages at first sex
between the cases and the controls. Before undertaking a formal test, it
might be useful to display the survival times graphically. One possibility
is to display individual survival times and indicate the censoring in the
plot in some way. Such a plot can be created in SPSS by constructing a
drop line chart. The necessary instructions are as follows:
Rearrange the spreadsheet rows in ascending order of sexstat and
agesex by using the commands Data – Sort Cases – Sort by: sexstat (A)
agesex (A) – Sort Order Ascending – OK.
Use the Compute command (see Chapter 1, Display 1.13) to generate
a new variable (labeled start) that contains zero values.
Employ the Split File command to organize all further output by
diagnostic groups (see Chapter 1, Display 1.10).
The commands Graphs – Line… – Drop-line – Values of individual cases –
Define open the Define Drop-line: Values of Individual Cases dialogue box
where the variables agesex, and start need to be listed under the
Points Represent list and the variable sexstat as the Category Labels
Variable.
The resulting charts are shown in Display 10.1. It is apparent that
almost all censoring occurs among the cases. Although ages at first sex
appear younger in the control group based on the uncensored data, such
comparisons can be misleading when differential censoring is present. It
would help if we could display the distributions of the survival times in
© 2004 by Chapman & Hall/CRC Press LLC