[Rd] Shapiro Test P Value Incorrect? (PR#9768)

From: <automorphism_at_gmail.com>
Date: Fri, 29 Jun 2007 22:55:10 +0200 (CEST)


Full_Name: Jason Polak
Version: R version 2.5.0 (2007-04-23)
OS: Xubuntu 7.04
Submission from: (NULL) (137.122.144.35)

Dear R group,

I have noticed a strange anomaly with the shapiro.test() function. Unfortunately I do not know how to calculate the shapiro test P values manually so I don't know if this is an actual bug.

So, to produce the results, run the following code:

pvalues = 0;
for (i in 1:17)
{

	j = 1:(i+3);
	pvalues[i]=shapiro.test(j)$p;

}

plot(pvalues);
print(pvalues);

Now I just made the graph to illustrate that the p-values are strictly decreasing. To me this makes intuitive sense: we are using the Shapiro test to test normality of (1,2,3,4),(1,2,3,4,5), and so on. So the p-value should decrease.

These are the p-values:
[1] 0.9718776 0.9671740 0.9605557 0.9492892 0.9331653 0.9135602 0.8923668
[8] 0.8698419 0.8757315 0.8371814 0.7964400 0.7545289 0.7123167 0.6704457

[15] 0.6294307 0.5896464 0.5513749

However, there is an increase in p-value when you go from (1,..,11) to (1,..,12). Is this just a quirk of the Shapiro test, or is there an error in the calculation algorithm?



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