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Date:         Tue, 1 May 2007 17:35:08 +0200
Reply-To:     Marta García-Granero
              <biostatistics@terra.es>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Marta García-Granero
              <biostatistics@terra.es>
Organization: Asesoría Bioestadística
Subject:      Re: shapiro-wilks
In-Reply-To:  <12B8F708F6A7E94AAD1EFF9FC1B4B341022E860D@AZDESGOVEX001.azdes.gov>
Content-Type: text/plain; charset=ISO-8859-1

Hi Fermin

Monday, April 30, 2007, 6:17:07 PM, You wrote:

OF> I think there is room for confusion here.

OF> Ho: the distribution is non-normal, OF> Ha: the distribution is normal.

Ouch!

NO, NO, definitely NO. The null hypothesis for a normality test is that the variable IS normal (believe man, I'm a statistics teacher...). Null hypotheses, as I said in my previous mail, say that there are no differences, no effects... In this particular case, it says that the observed distribution is NOT different from the one we would expect had the sample been drawn from a normal population.

If p-value >> alpha then conclude Ha.

Ouch again!!

p-value >> alpha means "accept H0".

If p-value >> It may not be If p-value >> wise to give more weight to the graph especially if one If p-value >> is unfamiliar with the shapes of the distributions (long If p-value >> tails, short tails).

In big samples, tails are of very little importance (leptokurtosis effect dilutes faster with sample size than skewness, I even read a math demo of that effect time ago). Skewness, on the other hand, is important and is easily spotted with a histogram

OF> Hi Christian

OF> Saturday, April 28, 2007, 4:30:08 AM, You wrote:

CH>> What it the null hypothesis of shapiro wilks test of univariate CH>> normality. That is if p < .05, does this indicate normality, or non- CH>> normality?

OF> In general, the null hypotheses for any statistical test is "no OF> effect", "no differences". This means that for a normality test, the OF> null hypothesis is "No differences from a normal distribution". p<.05 OF> means NON-NORMALITY.

OF> Anyway, remember that the p-value is not really informative. Normality OF> tests have low power if sample size is low (don't use them for sample OF> sizes below 10-12 cases), and are over sensitive for vey big samples OF> (if n is bigger than 100 then take a look at the histogram with a OF> normality curve plotted over it and decide if the variable looks OF> normal enough).

OF> -- OF> Regards, OF> Dr. Marta García-Granero,PhD mailto:biostatistics@terra.es OF> Statistician

OF> --- OF> "It is unwise to use a statistical procedure whose use one does OF> not understand. SPSS syntax guide cannot supply this knowledge, and it OF> is certainly no substitute for the basic understanding of statistics OF> and statistical thinking that is essential for the wise choice of OF> methods and the correct interpretation of their results".

OF> (Adapted from WinPepi manual - I'm sure Joe Abrahmson will not mind)

OF> NOTICE: This e-mail (and any attachments) may contain OF> PRIVILEGED OR CONFIDENTIAL information and is intended only for OF> the use of the specific individual(s) to whom it is addressed. It OF> may contain information that is privileged and confidential under OF> state and federal law. This information may be used or disclosed OF> only in accordance with law, and you may be subject to penalties OF> under law for improper use or further disclosure of the OF> information in this e-mail and its attachments. If you have OF> received this e-mail in error, please immediately notify the OF> person named above by reply e-mail, and then delete the original OF> e-mail. Thank you.

-- Regards, Dr. Marta García-Granero,PhD mailto:biostatistics@terra.es 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)


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