The “Elucidation rate” is a metric severely criticized but used for years for assessing the French police performance and a core indicator within the dashboards of the police administration. The indicator number of “gardes à vue” (police custody) has been recently the subject of a fierce public debate The two metrics are linked by the hypothesis according to which the garde a vue is a major lever of the elucidation rate.Bringing out mix variances explains the greater part of the performance improvement and belies the rationality of the increase of “gardes à vue”.