Hi sorry for the delayed response, I was not able to reply during the last week.
You are right that the F-test provided in the LME toolbox initially does not provide information about the direction of the effects. You could do the following: First, it is always useful to plot the data to get a first impression. However, if the statistical model also contains additional covariates, this plot does not completely reflect the effects as estimated by the model. So this may not give a complete picture. A second option is to take a look at the sign of the estimated beta values, which also reflect the direction of the effects. These beta values are stored in the output of the 'lme_fit_FS' script, e.g. 'total_hipp_vol_stats.Bhat' in the tutorial example. For proper interpretation, you'd need to relate the beta values and their corresponding columns of the design matrix, and also take your contrasts of interest into account. So this is somewhat complicated. As an alternative, the LME toolbox also provides so-called "signed p- values". Note that these are not p-values in the conventional sense. A signed p-value is the sign (-1 or +1) of the scalar product of a row of the contrast matrix and the vector of the estimated betas, and it can give information about the direction of a specific effect. These signed p-values can be accessed from the output of the 'lme_F' script, see e.g. the 'F_C.sgn' variable for the tutorial example. There will be as many signed p-values as there are rows in the contrast matrix, so each signed p-value corresponds to one row of the contrast matrix. The interpretation of this signed p-value depends on how the contrast was formulated: In a hypothetical example (different from and simpler than the tutorial example), suppose that one row of a contrast matrix starts like 0 1 -1 ... and the entries correspond to group1, group2, group3, ..., then a positive signed p-value will in this particular case indicate that the difference 'group2 minus group3' is greater than zero, which means that group2 has greater values than group3. On the other hand, a negative signed p-value will in this particular case indicate that the difference 'group2 minus group3' is less than zero, and hence group3 must have greater values than group2. However, also consider the equally valid alternative that a row of a contrast matrix starts like 0 -1 1 ... and the entries again correspond to group1, group2, group3, ..., then a positive signed p-value will in this particular case indicate that the difference 'group3 minus group2' is greater than zero, which means that group3 has greater values than group2. On the other hand, a negative signed p-value will in this particular case indicate that the difference 'group3 minus group2' is less than zero, and hence group2 must have greater values than group3. So the second interpretation is just the opposite as the first one. This illustrates that careful attention needs to be paid to the formulation of the contrasts. Also, it's best if the interpretation gained from the signed p-values agrees with the interpretation of a simple plot of the data. To summarize, if we are testing for simple group differences, the F- test only provides a single p-value, which indicates if there is at least one significant difference among the several groups. To get an idea which groups actually differ, and in which direction, one needs to take a closer look, for example at those effects that are reflected in the rows of the contrast matrix; for this, one option is to use the signed p-values as described above. Hope this helps, Kersten On Di, 2018-03-27 at 15:31 +0200, lanbo Wang wrote: > Dear Kersten, > > I have a question about LME model. After I acquired p value, could I > know which group is bigger? > > Thanks, > Lanbo > > On Fri, Mar 16, 2018 at 12:13 AM, lanbo Wang <drram...@gmail.com<mail > to:drram...@gmail.com>> wrote: > Dear Kersten, > > Thanks a lot, it's really help. I have another question, after I got > results that two group have significant, then how could I get > direction? > > Thanks, > Lanbo > > On Tue, Mar 13, 2018 at 5:40 PM, Diers, Kersten /DZNE <Kersten.Diers@ > dzne.de<mailto:kersten.di...@dzne.de>> wrote: > Hello, > > On Di, 2018-03-13 at 21:48 +0100, lanbo Wang wrote: > > > > Dear Kersten, > > > > Thanks, I find it. And I have other questions: > > 1. The intercepts all set as one, so in this model it doesn't > > separate different subjects, or can say no individual subject > > change > > rate? > If I understood correctly, the question is whether or not we can get > estimates for individual slopes across time? > > If so, then yes, that's possible - but I'd have to run an analysis > myself and look up how to do it exactly - I'll get back on this. > > > > > 2. Should we set age according to different timepoint, or just use > > baseline age? > It's better to use age at baseline. > > One of the nice things of the LME is that it can separate the cross- > sectional effect of age (at baseline) and the longitudinal effect of > aging (=effect of time). So the aging effect is already incorporated > within the 'time since baseline' variable, which of course should > also > be present in the model. > > Since it is difficult to estimate and interpret effects that are very > redundant (such as time vs age at each timepoint), it's better to > just > use age at baseline for the other regressor. > > Best regards, > > Kersten > > > > > Thanks, > > Lanbo > > > > On Tue, Mar 13, 2018 at 3:34 PM, Diers, Kersten /DZNE > > <Kersten.Diers@ > > dzne.de<http://dzne.de><mailto:kersten.di...@dzne.de<mailto:Kersten > > .di...@dzne.de>>> wrote: > > Hello Lanbo, > > > > the univariate example data can actually be downloaded from the LME > > tutorial website: > > > > Search for: "An optional sample dataset which can be used to become > > familiar with the LME Matlab tools can be found here". The linked > > tar.gz archive contains two folders, one for the univariate and one > > for > > the mass-univariate example data. > > > > Best regards, > > > > Kersten > > > > On Di, 2018-03-13 at 13:38 +0100, lanbo Wang wrote: > > > > > > > > > Dear Experts, > > > > > > Hi, > > > There is no example detail on website of LME tutorial. I have > > > some > > > question about it. > > > Could you send me the table of ADNI univariate example data? > > > > > > > > > Thanks, > > > Lanbo > > _______________________________________________ > > Freesurfer mailing list > > Freesurfer@nmr.mgh.harvard.edu<mailto:freesur...@nmr.mgh.harvard.ed > > u><mailto:Freesurfer@nmr.mgh.harvard.edu<mailto:freesur...@nmr.mgh. > > harvard.edu>> > > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > > > > > The information in this e-mail is intended only for the person to > > whom it is > > addressed. If you believe this e-mail was sent to you in error and > > the e-mail > > contains patient information, please contact the Partners > > Compliance > > HelpLine at > > http://www.partners.org/complianceline . If the e-mail was sent to > > you in error > > but does not contain patient information, please contact the sender > > and properly > > dispose of the e-mail. > > > > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu<mailto:Freesurfer@nmr.mgh.harvard.edu> > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
