Biostatistics questions

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biobio3012
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Biostatistics questions

Post by biobio3012 » Mon May 08, 2006 2:50 pm

I am not sure whether violoation of the assumptions of an analysis will or will not have influence on p-value?
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canalon
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Post by canalon » Mon May 08, 2006 3:57 pm

Violations of assumptions means that you are applying a test that you shouldn't be applying to your data. The p-value may still be computed, but you cannot interpret it.

For example if your population has two maxima, and you are applying a t-test, which assumes a normal distribution, you may still get a p-value, but whatever result it doesn"t mean anything. This mean that you should probably use another test, here a non parametric one.
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

biobio3012
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Post by biobio3012 » Tue May 09, 2006 1:37 pm

so you are saying that the p-value may be influenced by the violation of assumption? I am not sure what I think is right, but I think that you use P value to reject/accept the assumption(hypothesis), so if the hypothesis is violated, it doesnt change p-value because p-value is set.
sincerely,

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canalon
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Post by canalon » Tue May 09, 2006 2:59 pm

No I am saying that even though you may always be able to compute a p-value, if you violated the assumptions of your test in the beginning (if for example it assumes a normal distribution of your results and it is not true for your data) you should not draw any conclusion from your result.
The interpretation of the p-value depends on the set of assumption in the beginning and can only be interpreted if you respect them. Find yourself a good biostatitics book if you want more explantions.
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)

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