Abraham, Manja D., Peter D.A. Cohen and Dirk Jan Beukenhorst (2001), Letter to the Editors. The British Journal of Psychiatry, Vol 179 2001, pp. 175-177.
© Copyright 2001 Manja Abraham. All rights reserved.


Letter to the Editors

Comparative cannabis use data

Abraham, M.D., P.D.A. Cohen and D.J. Beukenhorst

Robert MacCoun and Peter Reuter (British Journal of Psychiatry, 2001, 178, pp. 123-128) examine alternative legal regimes for controlling cannabis availability and use. They claim that the Dutch experience (the coffeeshop system with first decriminalisation of purchase, followed by "commercial promotion") significantly increases cannabis use prevalence. They conclude however that primary harm comes more from criminalisation than from decriminalisation. They base their conclusions on the comparison of cannabis use data from the Netherlands and from other countries. Rightly they warn that "meaningful cross-sectional comparisons of drug use should be matched for survey year, measure of prevalence. and age groups covered in the estimates". They forget that the comparisons should also be matched for type of geographical area. Comparing Flatbush to New York City would have limited relevance, even if matching for age group etc would be right. If one wants to compare cannabis use in New York City with somewhere else, one would have to look for a similar area, both in address density as in variation of its population and life styles. Amsterdam could be compared to San Francisco, because these cities are very similar in size and cultural characteristics, but not to New York City, a metropolis over 10 times its size, let alone do a comparison between Amsterdam and the US as a whole. Such comparisons are wrong and without meaning.

We agree with MacCoun and Reuter that decriminalising cannabis merits serious consideration but we disagree with their observations on "commercialisation" . In this letter we will turn most of our attention to the epidemiological material the authors base their conclusions on.

MacCoun and Reuter focus entirely on cannabis prevalence (assuming that a lower prevalence is better than a higher one) without considering whether this is the most relevant issue; the social and legal consequences of the use of cannabis could be considered at least as important. For a fuller understanding of policy options these consequences should be part of the comparison as well. Agreeing that a comparison of mere prevalence figures is a useful first step towards informed comparisons, we have the following thesis. The conclusion of MacCoun and Reuter that the commercial type of Dutch coffeeshop system increases cannabis prevalence is based on statistically ill-founded comparisons of Dutch prevalence figures to other Western nations.

MacCoun and Reuter find their empirical evidence in a comparison of 28 cannabis use prevalence figures. Cannabis prevalence figures of a Dutch city or Dutch nation-wide figures are compared to prevalence figures of the USA or other Western nations. Differences are summed up and averaged out, resulting in a/o a mean Dutch-US difference and a mean Dutch-European difference. This is statistically erroneous for reasons we will supply below.

1. In 16 cases a Dutch city is compared to a nation (UK, US, Sweden, etc). By doing this, MacCoun and Reuter presuppose that prevalence rates are the same all over the Netherlands. This is incorrect: in our 1997 national survey we found large geographical differences between locations with different address densities, a measure of urbanization, (e.g. the lifetime prevalence of cannabis use in Amsterdam -address density over 3000 per square km- was 36.7%, the average national prevalence was 15.6% and average prevalence in rural areas -address density less than 500 per Km2- 10.5%). Correct international comparisons can be made, but have to be between comparable geographical or urban areas. Despite the sensitivity MacCoun and Reuter demand for correct comparisons, nation wide USA figures (260 million inhabitants that include major metropolitan areas) are compared to the small Dutch city of Tilburg (165 000 inhabitants).

2. Comparisons are arbitrarily selected. For example: by replacing prevalence figures of Amsterdam (the city most often chosen in MacCoun and Reuter's comparisons) by figures of Rotterdam changes the outcomes of the average difference in cannabis prevalence between the Dutch and other systems.

3. MacCoun and Reuter state that the lifetime prevalence of cannabis in the Netherlands has increased consistently and sharply in the age group 18-20. According to MacCoun and Reuter "the increase provides the strongest evidence that the Dutch regime might have increased cannabis use among the young". This finding is based on school survey data (lifetime cannabis use in 1984: 15%, in 1996: 44%). Again the choice of figures that are compared is crucial. Moreover, the Dutch school survey data of the age group of 18-20 is an extremely biased selection of this age cohort. The school survey takes place in some primary schools, but mostly in secondary educational institutions, that are designed for the 12-18 year old. However, some persons remain much longer in this system by a variety of reasons but they are atypical for the age group in general. They bias the school survey estimate for this age group. More suitable figures are given by Statistics Netherlands (CBS) and by the Centre for Drug Research (Cedro), and reflect a much more moderate increase or no increase at all. Statistics Netherlands measures cannabis use prevalence in a national representative sample. For the age group 18-20 lifetime cannabis use remains at the same level over time (17% in 1989, 19% in 1990, 18% in 1991, 20% in 1992 and 14% in 1993). Using Cedro data, we are able to produce trend data for the city of Amsterdam for the same age group 18-20: in 1987 lifetime cannabis use was 34%, rising tot 44% in 1997. This is a rather modest increase in cannabis use, very similar to slowly rising consumption levels of other European and US measurements. The age group of 18-20 year olds in the samples of Amsterdam are randomly selected from the citizen registry, and represent the age group much better than 18-20 year olds still attending school. The "dramatic" increase that MacCoun and Reuter hypothesize in Dutch cannabis use in the period 1984 to 1996 (as reflected in the same age group) does not exist!

4. We maintain that many of the comparisons that MacCoun and Reuter propose are incorrect because of the reasons stated above. But, the most serious flaw develops by creating a series of "absolute" differences between Dutch and other data, and averaging them. MacCoun and Reuter create the suggestion that too large or too small differences will be averaged and thereby, in the form of an "average" difference, become more reliable. The opposite is true. If pears can not be compared to apples, their "differences" can not be used for normal mathematical computations.


Abraham, M.D., P.D.A. Cohen, R.J. van Til, M.A.L. de Winter
1999 Licit and illicit drug use in the Netherlands. Amsterdam: CEDRO.

Centraal Bureau voor de Statistiek
POLS (unpublished time series).

Substance Abuse and Mental Health Services Administration
1997 National Household Survey On drug abuse: Population Estimates 1997. Department of health and human services. Rockville, DHHS Publication.

See also:

Authors' Reply by MacCoun, R., P. Reuter
2001 Comparative cannabis use data. The British Journal of Psychiatry 179: 175-177. The Royal College of Psychiatrists.