Re: Chi suqare problem - large cells number and significant test validity

Posted by Bruce Weaver on Mar 25, 2011; 1:38pm
URL: http://spssx-discussion.165.s1.nabble.com/Chi-suqare-problem-large-cells-number-and-significant-test-validity-tp4210454p4264111.html

Glad to hear you found it helpful.  

A couple points re Fisher's exact test (aka the Fisher-Irwin test).  First, it is designed for the situation where all of the marginal totals are fixed.  If all marginal totals are fixed in advance, then use FET regardless of sample size.  But in practice, this situation does not arise very often, it seems to me.  One example would be where you have a fixed number of cases in two groups (e.g., males and females), and then do a median split on some continuous variable to obtain the other dichotomy.  But doing a median split throws away a lot of info, and some other method (e.g., t-test or logistic regression) would generally be better.  

The more common use of FET is for dealing with expected counts < 5.  But in most of those cases, you'd be better off using the N-1 chi-square, IMO.  For more info, scroll down to "2x2 Tables: Advice from Campbell (2007)" on that webpage I gave last time.  See also Campbell's nice website, that includes a calculator for the N-1 chi-square.

  http://www.iancampbell.co.uk/twobytwo/calculator.htm

HTH.

chengfoh wrote

Hi Bruce:

Thanx so much for your advices, it is very helpful for me. Now i shall proceed with the statistics.

Also, they do apply fisher\s exact test when the sample size is small, but does it matter if i use it on large sample size at n =150?


Cheng Foh