Estimating UK Consumption from Seizures and Busts

Another way of estimating national consumption from our data starts by assuming that the proportion of respondents who had been convicted or cautioned for cannabis offences (21% - Table 26) was similar to that among all regular cannabis users.

As in other markets, a small proportion of people buying larger quantities account for a disproportionate amount of the cannabis which is sold. As well as buying more, they are doing so at lower unit prices. Some will be consumed, some sold on. Although the proportion of transactions reported which were under 1/8oz was slightly higher than the proportion over 1oz, after "weighting" according to the estimated sizes and unit prices of deals, the former account for only 0.33% of the total market, and the latter for over 40%. The price per gram can be "weighted" according to the same factors, to assist in estimating the total economic value of the cannabis market. (Table 25)

Questions used the used the term "bust" which is common parlance, to mean a conviction or caution for any offence. It is possible that some respondents took it to include "not guilty" verdicts, dropped cases, or arrests or searches not leading to prosecution, which would lead to the proportion reporting busts being high. We have assumed one seizure per bust.

However, extrapolations from our data are unlikely to represent the general population due to the specific nature of the groups questioned (almost all regular cannabis users at rock festivals). We have probably over-represented the proportions of regular as opposed to occasional users, since the situation was one where use was more likely than usual. The estimate of total number of UK users from respondents" "busts" would probably be conservative, as other research has shown that festival-goers are more likely to be "busted" than some other cannabis users.

Customs" seizures tend to be of larger quantities than the polices, mean 11.5kg and 286g respectively. Police data may be a better indicator of what quantities are on the market at what prices, though still distorted by small numbers of very large transactions.

The number of "busts" reported can be correlated with respondents" reported duration of use, frequency of purchasing and quantities usually bought, to reach estimates of the number of busts statistically likely per year for various levels of consumption. There were two questions on purchasing, and another on use, which are each given here and also averaged out. Estimates of the proportions being bought for personal use are made by comparing reported amounts used with amounts bought. (Table s 26 & 27)

From the amounts seized by police and Customs, the numbers of seizures, and the estimated numbers of busts per transaction, it is possible to estimate the total number of transactions represented, 135,826,030, and the quantity not seized, 2,874,078.8 kg.

Several estimates can then be made of the total value of the cannabis market, based on the price weighted by size of transaction, £3.21/g, or on a modal street price of £15 per 1/8oz (£4.29/g). The estimates vary according to the value and size of transactions, and the differing figures from responses to different questions on use levels. (Table 28 )

Using some of the estimates above for transactions per bust and for different consumption levels, and the numbers of people convicted or cautioned for cannabis offences in that year, numbers consuming at various levels and thus the size and value of the whole market can be estimated. The values range from £2,933 million to £524,479, much lower than estimates made by other methods.

Home Growing We have assumed that homegrown cannabis plants as estimated from police seizures produce 15 marketable grams each. Those respondents who had ever grown their own were more likely to have been busted. We have no information on what proportion took up cultivation after a bust. It is plausible that growers are more likely to be caught because plants are conspicuous. They were also significantly heavier users. Differences between reported amounts used and bought will have been partly due to home grown use and sale.

Table 24