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help > RE: ANCOVA - contrast / hypothesis testing
Jan 6, 2020 11:01 PM | Andrew Zalesky
RE: ANCOVA - contrast / hypothesis testing
Hi Athina,
1. The matrix in the text file is not full rank. The first column is clearly a linear combination of the second and third column. I don't think that this is a valid design matrix. One of the first three columns should be omitted - it does not matter which one.
4. Yes - removing ROIs from the network can certainly change the results. The effect of removing ROIs is difficult to predict a priori. If you remove a node the comprises the effect, this size of the subnetwork will shrink and this can potentially cause a loss of significance. If you remove a node that does not comprise the effect, this can potentially reduce the potential for large components to form as a matter of chance.
So, it is difficult to unequivocally say that removing ROIs will result improve power.
Andrew
Originally posted by Athina Aruldass:
1. The matrix in the text file is not full rank. The first column is clearly a linear combination of the second and third column. I don't think that this is a valid design matrix. One of the first three columns should be omitted - it does not matter which one.
4. Yes - removing ROIs from the network can certainly change the results. The effect of removing ROIs is difficult to predict a priori. If you remove a node the comprises the effect, this size of the subnetwork will shrink and this can potentially cause a loss of significance. If you remove a node that does not comprise the effect, this can potentially reduce the potential for large components to form as a matter of chance.
So, it is difficult to unequivocally say that removing ROIs will result improve power.
Andrew
Originally posted by Athina Aruldass:
Hi again Andrew - many thanks for your
clarification ! I do however have a couple of follow-up questions
re Q1 and Q4.
Q1 - The reason I queried this was because NBS did not actually generate a rank deficient warning / error when I performed the analysis with design matrix (b). I have appended the full .txt for you to inspect. Should it actually be complaining ?
Q4 - I was thinking perhaps a change in results more from no significant network to potentially an emergent subnetwork after restricting number of ROIs - Is this a possibility following reduction in multiple comparisons ? Please correct me, I think I am not understanding the mechanics of NBS - but is the likelihood of finding a statistically significant cluster dependent at all on the size of input correlation matrices / number of individual edges ? Is the statistical testing not performed on the resultant component ie cluster with each permutation, circumventing the multiple comparisons issue entirely ? Or would it actually effect outcome if size of cluster is set to extent instead of intensity (what I have been using) ?
Please advise. Sorry and many thanks (also Happy New Year) - Athina.
Q1 - The reason I queried this was because NBS did not actually generate a rank deficient warning / error when I performed the analysis with design matrix (b). I have appended the full .txt for you to inspect. Should it actually be complaining ?
Q4 - I was thinking perhaps a change in results more from no significant network to potentially an emergent subnetwork after restricting number of ROIs - Is this a possibility following reduction in multiple comparisons ? Please correct me, I think I am not understanding the mechanics of NBS - but is the likelihood of finding a statistically significant cluster dependent at all on the size of input correlation matrices / number of individual edges ? Is the statistical testing not performed on the resultant component ie cluster with each permutation, circumventing the multiple comparisons issue entirely ? Or would it actually effect outcome if size of cluster is set to extent instead of intensity (what I have been using) ?
Please advise. Sorry and many thanks (also Happy New Year) - Athina.
Threaded View
| Title | Author | Date |
|---|---|---|
| Athina Aruldass | Dec 6, 2019 | |
| Athina Aruldass | Dec 18, 2019 | |
| Andrew Zalesky | Dec 19, 2019 | |
| Athina Aruldass | Jan 6, 2020 | |
| Andrew Zalesky | Jan 6, 2020 | |
| Athina Aruldass | Dec 10, 2019 | |
| Andrew Zalesky | Dec 13, 2019 | |
| Athina Aruldass | Dec 9, 2019 | |
| Andrew Zalesky | Dec 7, 2019 | |