= 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'.

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.