cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 
JMP is taking Discovery online, April 16 and 18. Register today and join us for interactive sessions featuring popular presentation topics, networking, and discussions with the experts.
Choose Language Hide Translation Bar
Mananahi
Level I

Dunnett's Test Statistic

I have run an ANOVA in JMP with a Dunnett's post hoc test to compare 8 treatments to my control. In a publication I am working on I reported the p-values for each comparison to the control. A reviewer has asked me to include the test statistic along with the p-values. For some reason, a test statistic is not displayed along with the p-values in the default settings on the Dunnett's output within the fit model personality. I'm new to statistics so I'm not even certain what the test statistic is for a Dunnett's procedure.  Does anyone know what that test statistic would be and how I can get JMP to display it?

2 ACCEPTED SOLUTIONS

Accepted Solutions
Thierry_S
Super User

Re: Dunnett's Test Statistic

I believe what you are looking for is the Q value shown in the LSMeans Difference Dunnett table (equivalent to t statistics but for multiple comparison to a control value).
If this not what you are looking for, then I'm afraid that I will not be able to help you any further.
TS
Thierry R. Sornasse

View solution in original post

David_Burnham
Super User (Alumni)

Re: Dunnett's Test Statistic

I've put together some fake data to illustrate what is going on:

 

dunnetts.PNG

 

I've chosen the data such thast the contrast between c and the control (a) has a p-value of 0.05 i.e. the threshold at which we are assessing statistical significance.  

 

A t-statistic can be calculated as a the ratio of the difference to the standard error of the difference.  For the second contrast the statistic is 2.5 i.e. the Q statistic.  So whilst there is a statistic for each comparison, we can define a single statistic to that is the threshold for statistical significance.

 

You can check Wiki for references on the method although there is a tendency for Wiki to be overly abstract for statistical methods but you should easily be able to search for other explanations, e.g.

http://davidmlane.com/hyperstat/B112114.html

-Dave

View solution in original post

6 REPLIES 6
Thierry_S
Super User

Re: Dunnett's Test Statistic

Hi, 

 

I think you may be able to get what the reviewer is asking by performing your test in the "Fit Model" analysis instead of the "Y by X".

 

Here is a simple example with dummy data (Y) for 3 categorical levels (X)

 

Go to Analysis > Fit Model and assign your Y and X as shown:

 Fit Model Simple ONEWAY.PNG

Click Run, and in the model result, go to GROUP effect > Dunnett's

Sub Menu Dunnet's in Fit Model Result.PNG

The resulting screen in giving you the full test statistics (LS Means Differences Dunnett's) as shown in this final screenshot:

Dunnet Test Statitics in Fit Model.jpg

That should satisfy the reviewer. If it does not, you should try to clarify what he/she means by Test Statistics.

 

TS

Thierry R. Sornasse
Mananahi
Level I

Re: Dunnett's Test Statistic

So I'm looking at the LS Means Differences Dunnett's table on your screenshot and it has columns for Level, -Level, Difference, Std Err Dif, Lower CL, Upper CL, and p-Value. Shouldn't there be one more column in there that says t-Statistic (or some other test statisctic) next to the p-value so that we know what was used to calculate that p-value. Something like that seems to be on most JMP outputs. That's what I think I am looking for, but maybe I just don't understand stats and am assuming that something exists. Do Dunnett's procedures not have test statistics?
Thierry_S
Super User

Re: Dunnett's Test Statistic

I believe what you are looking for is the Q value shown in the LSMeans Difference Dunnett table (equivalent to t statistics but for multiple comparison to a control value).
If this not what you are looking for, then I'm afraid that I will not be able to help you any further.
TS
Thierry R. Sornasse
Mananahi
Level I

Re: Dunnett's Test Statistic

I think that is what I'm looking for. thanks. I'm still a little confused though. Why is there only one statistic for the whole table. Most tables have an individual test statistic for each p-value. Can you or anyone enlighten me to why Dunnett's would be different?
David_Burnham
Super User (Alumni)

Re: Dunnett's Test Statistic

I've put together some fake data to illustrate what is going on:

 

dunnetts.PNG

 

I've chosen the data such thast the contrast between c and the control (a) has a p-value of 0.05 i.e. the threshold at which we are assessing statistical significance.  

 

A t-statistic can be calculated as a the ratio of the difference to the standard error of the difference.  For the second contrast the statistic is 2.5 i.e. the Q statistic.  So whilst there is a statistic for each comparison, we can define a single statistic to that is the threshold for statistical significance.

 

You can check Wiki for references on the method although there is a tendency for Wiki to be overly abstract for statistical methods but you should easily be able to search for other explanations, e.g.

http://davidmlane.com/hyperstat/B112114.html

-Dave
David_Burnham
Super User (Alumni)

Re: Dunnett's Test Statistic

Also I should have mentioned JMP's online help:

dunnetts2.PNG

https://www.jmp.com/support/help/14-2/effect-details.shtml#625536

 

-Dave