The TESTVAL
subcommand invokes the One Sample mode.
This mode is used to test a population mean against a hypothesized
mean.
The value given to the TESTVAL
subcommand is the value against
which you wish to test.
In this mode, you must also use the /VARIABLES
subcommand to
tell PSPP which variables you wish to test.
A researcher wishes to know whether the weight of persons in a population
is different from the national average.
The samples are drawn from the population under investigation and recorded
in the file physiology.sav.
From the Department of Health, she
knows that the national average weight of healthy adults is 76.8kg.
Accordingly the TESTVAL
is set to 76.8.
The null hypothesis therefore is that the mean average weight of the
population from which the sample was drawn is 76.8kg.
As previously noted (see Identifying incorrect data), one
sample in the dataset contains a weight value
which is clearly incorrect. So this is excluded from the analysis
using the SELECT
command.
get file='physiology.sav'. select if (weight > 0). ttest testval = 76.8 /variables = weight. 
Results 15.2 shows that the mean of our sample differs from the test value by 1.40kg. However the significance is very high (0.610). So one cannot reject the null hypothesis, and must conclude there is not enough evidence to suggest that the mean weight of the persons in our population is different from 76.8kg.
