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| [wiki:[:FAQ/ChiSquaredSpss:CROSSTABS] in SPSS, consequently, outputs how many expected cell frequencies are less than five whenever a chi-square test is performed. Fisher's exact test may be requested by pressing on the exact button in the crosstabs dialogue box and requesting 'exact'. | [wiki:FAQ/ChiSquaredSpss CROSSTABS] in SPSS, consequently, outputs how many expected cell frequencies are less than five whenever a chi-square test is performed. Fisher's exact test may be requested by pressing on the exact button in the crosstabs dialogue box and requesting 'exact'. Alternatively the program fishrc computes this test on a UNIX machine. |
Exact test vs chi-square?
Howell (2002) on p.158 explains a difficulty using the Pearson chi-square test for testing the independence of frequencies in two-way tables. Namely that when the expected cell frequencies are less than five the Pearson statistics does not follow a chi-square distribution.
Instead it is recommended that Fisher's exact test is used when any expected frequency is less than five.
[wiki:FAQ/ChiSquaredSpss CROSSTABS] in SPSS, consequently, outputs how many expected cell frequencies are less than five whenever a chi-square test is performed. Fisher's exact test may be requested by pressing on the exact button in the crosstabs dialogue box and requesting 'exact'. Alternatively the program fishrc computes this test on a UNIX machine.
The p-value can then be quoted. This p-value is always two-sided except in the case of a 2 by 2 table where it can also be one-sided.
Reference
Howell DC (2002) Statistical methods for psychology fifth edition: Duxbury Press:Pacific Grove, CA.