"Based on the Space statistics (2011) 36.3% are sentenced for drug offences. This is a higher percentage compared to the latest data of the statistics of the Directorate-general showing that 31.3% of all the prison population is detained for drug offences (regardless their legal status).
"Space statistics for Belgium point to double counting since the main offence rule is not well defined in Belgium (Aebi and Delgrande 2011, p. 66). Consequently, this double counting also occurs with the latest data of the Directorate-general and cannot explain this higher percentage. Moreover, Space only includes sentenced prisoners, expecting not a higher, but lower percentage compared to the latest data of the Directorate-general, since the latter involves all categories of legal status. Compared to the self-reported data however, these figures show again a different result with 37.9% declaring to be in prison due to a drug offence (Van Malderen et al, in press). This item is examined by asking prisoners whether they are in prison for ‘drug possession’, ‘drug trade’, ‘for other reasons’ or ‘for other reasons and drugs’. Prisoners could give multiple answers. However, one would expect a result more in line with the 31.3 % of the Directorate-general since the same population is involved (all categories of legal status included) and also here double counting occurs.
"Nevertheless, taking into account the different data sources, one can observe that more than one in three is detained due to a drug offence. A distinction has to be made, however, between a drug offender and a drug using or drug-addicted offender. It is found repeatedly in 2008 and 2010, that the nature of the offence is not a predictable factor for drug use during imprisonment (Van Malderen et.al, in press). Both drug offenders and non-drug offenders take drugs while in prison and vice versa."


Deprez, Nathalie, et al., "2011 National Report (2010 data) to the EMCDDA by the Reitox National Focal Point" (Brussels, Belgium: OD Public Health and Surveillance, Scientific Institute of Public Health, October 2011), WIV-ISP/EPI REPORTS N° 002, pp. 223-224.