Posted by
Marta García-Granero on
Sep 05, 2006; 5:57pm
URL: http://spssx-discussion.165.s1.nabble.com/Query-re-authenticity-of-p-values-for-chi-square-goodness-of-fit-test-tp1070734p1070739.html
Hi Margaret
DL> I would be most grateful for some advice on whether or not to accept the
DL> p-value obtained from running the chi-square goodness-of-fit test in SPSS
DL> when the expected count is less than 5. I am familiar with the golden rule
DL> for when to use Fisher's Exact test for cross-tabulated data but what should
DL> one do in the one-dimensional case? When the expected count is less than 5,
DL> a p-value is generated by SPSS but this is accompanied by a footnote
DL> indicating that all expected counts are less than 5 (with no advice as to
DL> why this is important to know).
Cochran's rules on chi-square tests (all that compare Observed vs
Expected frequencies using: Sum[(Obs-Exp)**2/Exp]) state that a
minimum expected frequency of 5 for every cell is needed for
asymptotic p-values to be valid. Anyway, this condition can be relaxed
a bit: a minimum expected frequency of 1 is accepted as long as less
than 20% of the cells have such low frequencies. Outside these
conditions (tight or relaxed) asymptotic testing is not valid.
Now, you say that ALL cells have a low expected frequency (below 5).
This can mean that either you have too many categories (and should
try, as Dominic mentioned, to collapse them in a meaningful way, if it
is possible), or an overall sample size too small for reliable
asymptotic testing. I suppose that you don't have the EXACT TESTS
module installed? That would allow you to get an exact p-value,
independent of the minimum expected frequency limitation (I do have it
installed, and, if you want, I can compute the exact p-value for you
if you send me the observed frequencies in a private mail).
--
Regards,
Dr. Marta García-Granero,PhD mailto:
[hidden email]
Statistician
---
"It is unwise to use a statistical procedure whose use one does
not understand. SPSS syntax guide cannot supply this knowledge, and it
is certainly no substitute for the basic understanding of statistics
and statistical thinking that is essential for the wise choice of
methods and the correct interpretation of their results".
(Adapted from WinPepi manual - I'm sure Joe Abrahmson will not mind)