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Apr 22, 2013 1:30 PM
(2370 views)

Hi,

If there is anyone out there who can explain the difference between the p-values in JMP and the value calculated with the "T-test" function in excel, I would be very grateful. I will give the following explanation to elucidate the quandary that I face.

My experiment consisted of the mass of 82 different samples. This sample set consisted of 4 separate groups (labeled 1,3,4,5) with each group consisting of 25 different samples. When I read the "ordered difference report" from the XY graph measurement, I get the following p-values

However, when I do the same test on the raw data in excel (formatted as the table below)

using the "T-test" function in excel with the following format "=TTEST(A2:A9,A10:A25,2,2)" (choosing a test between group #1 and group #3) the value I get is much different than the one I get in JMP (0.2549). Can anybody give a reason what the difference might be?

Thanks,

Tei

6 REPLIES

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Apr 22, 2013 1:42 PM
(1960 views)

That depends partly on the options you selected in JMP.

See the doc and clarify which options you choose.

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Apr 22, 2013 2:53 PM
(1960 views)

Sorry for not specifying earlier... We used the "Each Pair, Student's T-test" choice. However, I tried different tests and it didn't seem to make a tremendous difference...

Thanks,

Tei

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Apr 23, 2013 9:56 AM
(1960 views)

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Apr 23, 2013 10:32 AM
(1960 views)

**ToddS**: I believe you are correct. I have investigated this issue before and came to a similar conclusion.

**Tei**: If you run the same t-test in JMP on just the subset of data containing groups 1 and 3 you should be able to confirm that the p-values between Excel and JMP agree in this limiting case.

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Apr 23, 2013 10:39 AM
(1960 views)

Hi Guys,

I would like to conclusively determine which statistic to believe as it might tell us if something "(possibly) works" or "(probably) won't". What statistic would you believe? Is there any better way to determine a better way of answering this logical statement using the above example as a reference?

Thanks,

Tei

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Apr 23, 2013 12:27 PM
(1960 views)

Tei,

You should consider a different method other than t-test, which has its own limitations. I know Tukey is often considered superior for comparing means with more than 2 groups.