02-18-2013 12:44 PM
I ran an exact test on a data set and got this:
|Statistics for Table of Testing by Years|
|Likelihood Ratio Chi-Square||16||11.7345||0.762|
|WARNING: 59% of the cells have expected counts less |
than 5. Chi-Square may not be a valid test.
|Fisher's Exact Test|
|Table Probability (P)||2.86E-10|
|Pr <= P||.|
|Sample Size = 241|
Does anyone know what the '.' for the Pr <= P mean? I can't find any information about this result on-line.
02-18-2013 01:27 PM
If you specified option MAXTIME= in your EXACT statement, then the missing probability means that the exact probability computation didn't have time to complete within the limit that you set. - PG
02-19-2013 07:29 AM
Continuing down this road, if you did NOT specify MAXTIME=, then you are likely in the problem area outlined in the documentation under Computational Resources (see "Also for a fixed sample size, time and memory requirements increase as the marginal row and column totals beome more homogeneous.") Looking at the asymptotic tests, the various chi-squared values are all less than the degrees of freedom, which is an indicator of this kind of homogeneity. The documentation recommends using Monte Carlo estimation in these situations.
However, there isn't any real indication of differences, even though the chi-squared probability values may be misleading due to the number of low-count cells. The simple ratio of chi-squared/df < 1 is a strong indicator of no difference.