* Computes the sample standard unit (z-score). Used to compute 'value' in
* terms of standard units. Note that 'value', 'mean' and 'sd' must be all
* from the same sample data.
* Computes the sample standard unit (z-score). Used to compute 'value' in
* terms of standard units. Note that 'value', 'mean' and 'sd' must be all
* from the same sample data.