Nonparametic tests ARE simply parametric tests applied after a (rank) transformation of the data.
You have a problem with heterogeneity of variance? Just use the separate variances t test rather than the pooled variances t test. It has been around since the 1940's, but most psychologists seem not to be aware of it. So sad. Major stat packs provide it whenever you do a t test. More than two groups? Use a Welch test. Your stat pack probably provides it as an option when you do ANOVA. Significance testing variances or normality prior to selecting a test? VERY POOR PRACTICE. The parametric tests will be most sensitive to violations of their assumptions when sample sizes are small, but in that case the tests of assumptions will have so little power that you will not be able to detect serious problems. With larger sample sizes the parametric tests will be more robust to such assumptions and the tests of such assumptions will have so much power that they will detect as significant deviations from equal variances or normal distributions that are so minor as to be of absolutely no consequence. Cheers, Karl W. http://core.ecu.edu/psyc/wuenschk/KLW.htm ----- Original Message ----- From: "Daniel Leucuta" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Wednesday, April 12, 2006 3:23 PM Subject: Re: Dilemma about testing normality, and equality of variance to check assumptions for tests before applying them Thanks everyone for your precious help!!! I really enjoyed, the answers and find them useful. They revealed to me a part of the beauty of statistics :) A side question relating to the one asked is: what about transforming non-normal data and testing it with parametric tests. Is this a better approach than using non-parametric tests? What are implication to the interpretation of the results? Kind regards!!! Daniel Subscription, posting, archives, etc. can be found at http://lists.psu.edu/archives/edstat-l.html --- To make changes to your subscription go to: http://acsun.frostburg.edu/cgi-bin/lyris.pl?enter=tips&text_mode=0&lang=english
