A number of remarks need to be made concerning this data, which is made up from statistics provided by the institutions inspected. a/ In the first place, the left-hand column requires explanation. The acronym PI refers to “persons implicated”, that is to say suspected of having committed offences or arrested while committing offences, stopped for questioning and then taken to the station by a police squad. Only some of them are placed in police custody (PC) by the senior law-enforcement officer (OPJ) (articles 62-2 and 63 of the Code of Criminal Procedure (Code de procédure pénale)). Under ordinary law24 police custody is fixed at 24 hours but can be extended by an equivalent period by a public prosecutor at a court of first instance: the table therefore mentions the portion of cases of police custody extended beyond twenty-four hours. In police custody, minors25 on the one hand, and persons committing traffic offences26, on the other, can be set apart: as shown by the headings “Minors/PC” and “Road traffic offences/PC”. b/ In the second place, the figures are calculated in a number of different manners. The rate of persons implicated and the rate of police custody, given in “x” per thousand, place the absolute numbers of persons implicated and persons placed in police custody in proportion to the population of the district coming under the authority of the police station inspected. Strictly speaking, persons who cannot be subject to police custody measures, and children under ten years of age in particular, should be subtracted therefrom. These figures are therefore approximate. The ratio between the number of cases of police custody and the number of persons implicated is based upon precise data. The national average comes to 49.4 % for 2009 and 45.6 % for 2010. However, as indicated in the note above, the figures for traffic-related offences are not included in this national data. If the figures were exhaustive, the “PC/PI” percentage would probably be closer to 60%. Establishment of the “Minors/PC” and “Road traffic offences/PC” ratios does not pose any special difficulties. The same applies to the proportion of cases of police custody of more than 24 hours as compared with the total number of cases of police custody. The number of cases of police custody per day is based on the total number of cases of police custody divided by the number of days in the year. Finally, the average length of police custody is not calculated on the basis of the absolute total of hours: indeed, a calculation of this kind would be much too cumbersome in view of the number of placements in police custody; moreover, in too many registers, as the Contrôle Général has noted on many occasions, the time of ending of police custody is often omitted. The average length shown above is calculated from a (random) sample of between twenty and thirty official records of the end of police custody, which the inspectors systematically ensure are handed over to them; these official records almost always include the times of commencement and ending of police custody. c/ In the third place, and above all, a few remarks are required concerning these figures. As far as the rate of persons implicated is concerned, the differences vary from 11.2 in a thousand to 84 in a thousand, which is clearly considerable. However, this difference is deceptive. Indeed, there is no reason to assume a priori that all of the persons implicated live in the police district: the higher the flows of movement in the 24 With regard to organised crime and delinquency, the period can be brought up to seventy-two hours (article 706-88 of the Code of Criminal Procedure). 25 Minors can be placed in police custody from thirteen years of age. However, from ten years of age, children suspected of having committed serious crimes or offences can be “detained at the discretion” of an OPJ for a twelvehour period, which may be renewed once (article 4 of the statutory instrument (ordonnance) of 2nd February 1945). 26 These persons are not included in the statement (État 4001) in which the police list their activity. 31