"• Twenty-three Police Forces in England and Wales participated in the study. Forces were requested to submit samples confiscated from street-level users. In early 2008, they submitted 2,921 samples for analysis to either the Forensic Science Service Ltd (FSS) or LGC Forensics at Culham (LGC F).
"• Initial laboratory examination showed that 80.8% were herbal cannabis and 15.3% were cannabis resin. The remaining 3.9% were either indeterminate or not cannabis.
"• Microscopic examination of around two-thirds of the samples showed that over 97% of the herbal cannabis had been grown by intensive methods (sinsemilla). The remainder was classed as traditional imported herbal cannabis.
"• Regional variations were found in the market share of herbal cannabis. Thus North Wales, South Wales, Cleveland and Devon and Cornwall submitted proportionately fewer herbal cannabis samples, whereas Essex, Metropolitan and Avon and Somerset submitted proportionately more. These differences were statistically significant at the 0.1% confidence interval.
"• The mean THC concentration (potency) of the sinsemilla samples was 16.2% (range = 4.1 to 46%). The median potency was 15.0%, close to values reported by others in the past few years.
"• The mean THC concentration (potency) of the traditional imported herbal cannabis samples was 8.4% (range = 0.3 to 22%); median = 9.0%. Only a very small number of samples were received and analysed.
"• The mean potency of cannabis resin was 5.9% (range = 1.3 to 27.8%). The median = 5.0% was typical of values reported by others over many years.
"• Cannabis resin had a mean CBD content of 3.5% (range = 0.1 to 7.3%), but the CBD content of herbal cannabis was less than 0.1% in nearly all cases.
"• There was a weak, but statistically-significant, correlation (r = 0.48; N = 112; P < 0.001) between the THC and the CBD content of resin."


Hardwick, Sheila; King, Leslie, "Home Office Cannabis Potency Study 2008," Home Office Scientific Development Branch (Sandridge, St Albans, UK: May 2008), p. 1.