[FieldTrip] ft_mergealgin: high residual variance?

[Maria Hakonen]

Dear FieldTrip experts,

I have run ft_mergealign across subjects to align the head positions.
However, the residual variance between the original and the realigned data
seems to be high:

original -> template             RV 21232.46 %
original             -> original RV 36.96 %
original -> template -> original RV 9579.95 %

Could someone please let me know what would be the largest acceptable
change in the residual variance, and what should I do if the residual
variance is too high? Does the increase in residual variance mean that
there is a large shift in the head position?

I have used ft_mergealign as follows:

template = list of subjects (i.e. I want to calculate an average head
position over the subjects)

grad = data.grad;
hs=ft_read_headshape([hdr_path nameList{subj} '/hs_file']);
vol = ft_headmodel_localspheres(hs,grad);

cfg = [];
cfg.template = template;
cfg.inwardshift = 1.0;
cfg.vol =  vol;
data_aligned = ft_megrealign(cfg, data);

Best,
Maria
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