Hi Paola,

the official answer is: no one knows.

The practical answer is: it should be OK to take only the first 3 time point without creating the base. The base is mainly used as an unbiased space for initializing the longitudinal processing. The base does not change much by adding a 4th time point and so this should be fine.

A few points:

- if your datasets come from different scanners or use different image protocols, you will have other biases between the datasets (not just the number of time points)

- if you are doing a group analysis and the new dataset (with 3 time points) has all groups (so that the 3 vs 4 time point situation is balanced), it would be best to use all time points of all datasets and do a LME analysis. Probably that is the best idea in any case. Why drop a valuable time point in dataset1?

Best, Martin

Dear all

 

I already ran the Freesurfer longitudinal stream on 4 timepoints from a set of subjects (dataset1).

I would like to perform a new analysis, merging these subjects with another subjects’ dataset (dataset2), which has only 3 timepoints. For this second analysis, I will use just the first 3 timepoints of dataset1. Should I re-run the longitudinal stream on the first 3 timepoints of dataset1, or is it fine if I use the output I already have (obtained using a template with 4 timepoints)?

Thanks for any suggestion

Best regards

Paola

 

 

 

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