Brian S. Everitt A Handbook of Statistical Analyses using SPSS

Table 3.6 Recovery from Depression after Treatment
Not depersonalized
patients
Source: Everitt, 1992.
Depersonalized Patients
Recovered Not Recovered
Recovered 14 5
Not recovered 2 2
as “recovered” or “not recovered” with the results shown in Table 3.6. Is
depersonalization associated with recovery?
3.4.2 Drug Treatment of Psychiatric Patients: Exact Tests
for Two-Way Classifications
The chi-squared test for independence between two categorical variables
uses a normal approximation to generate a p-value. The approximation
is appropriate when the expected value under independence is large
enough in each cell (say at least 5). For 2 2 tables, this assumption can
be avoided by using Fisher’s exact test. However, for r c tables, SPSS
does not supply an exact test by default. Exact tests can be generated for
any size cross-tabulation by permutation (e.g., for details, see Manly, 1999).
SPSS provides permutation tests for all test statistics used for two-way
classifications after clicking the Exact… button in the Crosstabs dialogue
box and checking Exact or Monte Carlo in the resulting sub-dialogue box.
The procedure is computationally intensive. SPSS sets a time limit and
offers to sample a set of permutations (Monte Carlo option). In the latter
case, the precision of the p-value is evaluated by a confidence interval.
Table 3.7 gives a cross-classification of a sample of psychiatric patients
by their diagnosis and whether they were prescribed drugs for their
treatment (data taken from Agresti, 1996). Display the cross-tabulation
graphically. Use an exact test to assess whether diagnosis and drug
Table 3.7 Drug Treatment by Psychiatric Patient Type
Drug Treatment
Drugs No Drugs
Patient type Schizophrenia 105 8
Affective disorder 12 2
Neurosis 18 19
Personality disorder 47 52
Special symptoms 0 13
© 2004 by Chapman & Hall/CRC Press LLC