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You can use the same format of protocol id for the metric as for MRI and physiology; because studies are selected by type, they an MRI and metric with the same protocol id will not get confused.
\[patient id][session number][yyyymmdd]
Table metric
'study_id' | 'int(11)' | 'NO' |
'covariate_value_id' | 'int(11)' | 'NO' |
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