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A "metric" is just a type of study associated with the subject, set in the subjects GUI. A separate very simple table records all covariate_values in that metric.
Protocol ID for metric
You can use the same format of protocol id for the metric as for MRI and physiology; because studies are selected by type, they 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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