Date:         Wed, 4 Apr 2007 10:41:04 -0400
Reply-To:     "Burleson,Joseph A." <burleson@up.uchc.edu>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         "Burleson,Joseph A." <burleson@up.uchc.edu>
Subject:      Re: Chi-squared and Chi-squared test for trend comparison
In-Reply-To:  A<200704041058.l34AkBUp027967@malibu.cc.uga.edu>
Content-Type: text/plain; charset="windows-1256"

Lou:

I would very highly advise to not use chi-square test of order when you have a (presumably) parametric, hopefully normally distributed variable such as age which is hypothesized to correlate with a dichotomous variable.

First test for skewness in the age distribution, and fix it by use of square root or natural ln if necessary.

In any event, simply use the much more powerful t-test to test the difference in the mean ages between the success and failure groups. Using chi-square seems unnecessarily complicated, with an actual loss of power. Partitioning age is arbitrary as to the number of categories, can get you in trouble if you have too few insofar as the relationship, and in trouble if you have too many insofar as power.

Joe Burleson

-----Original Message-----
From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of Lou
Sent: Wednesday, April 04, 2007 6:58 AM
To: SPSSX-L@LISTSERV.UGA.EDU
Subject: Chi-squared and Chi-squared test for trend comparison

Dear all,

I have used a chi-squared test and a chi-squared test for trend to analyse
data with ordered categories (e.g. success/failure in an exam against age,
partitioned into groups from youngest to oldest).  Success appears to
decrease with increased age.

My question concerns the difference between the chi-squared and the chi-
squared_trend values.  If the difference, which I understand to be a chi-
squared value, proves to be significant, what kind of things can I
deduce?  Do I deduce that the observed variation is not just due to a
linear trend and that a non-linear relationship exists?  Do I deduce that
other factors are influencing the outcome of success?  What other kinds of
analysis should I be carrying out to investigate further?

Any advice would be appreciated, just to point me in the right direction.
I realise that this a bit of a vague question, but I’m just trying to
learn about this stuff and never really have anyone on hand to ask.

Thanks in advance,

Lou