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
On 06/07/2017 03:51 PM, Valsasina Paola wrote:
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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