One of the problems with Stop & Search is the political aspect and another which may be more important but unquantifiable is the deterrent effect.
The fact that few stops end up with an arrest is to be expected. An impression gained of an individuals' perceived intention is a personal one. As a result an officer may believe a perfectly innocent person is 'up to no good' for the wrong reasons.
However, it has long been recognised that a Police presence deters crime; as a result it is possible that Stop & Search has such an effect which is by its very nature not quantifiable.
Whether any changes will either make the action more acceptable or effective is also unquantifiable at this stage.