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Date:         Mon, 7 Mar 2011 23:41:14 -0800
Reply-To:     Bridgette Portman <bportman@uci.edu>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Bridgette Portman <bportman@uci.edu>
Subject:      T-test or nonparametric test? Confused.
In-Reply-To:  <1299533931392-3413177.post@n5.nabble.com>
Content-Type: text/plain;charset=iso-8859-1

Hi everyone,

This is a rather elementary statistics question and I feel kind of stupid asking it. But I've managed to thoroughly confuse myself. I hope someone can help me out.

I've collected survey data from 260 respondents. As I'm analyzing demographic information, I have noticed that the distribution of ages in my sample is not normal. In fact, it is bimodal, with peaks around 20 and 60, and a trough around 40. This was due to my sampling method, not to any intrinsic pattern in the population I was sampling from. I want to be able to compare ages between various groups, such as men and women, in my sample. But can I use a t-test, given the abnormal distribution? Should I use a nonparametric test like Mann-Whitney U instead?

The reason I'm confused is that the bimodality is in my sample alone, do to my sampling technique. The ages in the population as a whole, I'm sure, has an underlying normal distribution. I am studying political activists, and in order to get at them, I sampled from a) college student political clubs, and b) actual political parties. The college kids tended to be around 20, while the party people were 50+. I know one alternative would be to just recode age into something like "below 40" and "above 40" but I'd rather avoid doing that if I can.

Can anyone offer advice?

Thanks, Bridgette

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