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Drug use in the general population is estimated through population surveys, based on representative probabilistic samples of the whole population under study. This methodology allows to survey drug use, patterns of use, and related factors (both potential determinants and consequences of use of drugs) directly for each individual under study. A number of factors can be investigated retrospectively, although with the limitations that self-report (concealment) and memory biases have on recall of past events.
In interpreting the results, one needs to bear in mind the limitations of surveys in estimating the more marginalised forms of drug use (e.g. heroin injection) due to non-probabilistic errors (exclusion from the sampling frame, absence, non-response).
Surveys provide estimates of the proportion of the population who have used different drugs during certain periods of time. For illegal drugs, the usual measures are:
any use during the person’s life (lifetime prevalence), often called ‘lifetime experience’ with drugs,
any use during the previous year (last-12-months prevalence), often called ‘recent use’ of drugs,
any use during the previous month (last-30-days prevalence), often called ‘current use’ of drugs.
Obviously, ‘lifetime experience’ always produces higher figures, but ‘lifetime experience’ alone will not capture well the current drug situation (among adults), as it is a cumulative measure that includes also people that tried drugs a long time ago. On the other hand, it can give a first rough estimation of the extent of drug experience in the population, valuable for those drugs of lower prevalence. Analysis by age group – or birth cohort – can give insight into the generational dynamics of drug use, when particular drugs became popular. It is helpful for estimation of incidence (year of first use), and for computation of continuation and discontinuation rates among those who have used drugs.
‘Recent use’ produces lower figures, but reflects better the present situation, giving an indication of recent but probably occasional use in most cases. The combination of lifetime experience and recent use can give basic information on drug use patterns (e.g. 'continuation rates').
‘Current use’ gives some indication of more regular use and will include the more intensive users, although in fact most current users will not be intensive users. The figures are generally low when the whole adult population (aged 15 to 64 years) is considered, except for cannabis.
Estimates of 'recent' or 'current' use could be substantially higher if analysis is focused on young people (15 to 24 or 15 to 34 years) particularly among males, and even more on urban areas. This focused analysis could be valuable for policy formulation and evaluation.
Many countries collect information on 'age of first use' of drugs, which allows analysis of incidence. Also intensity of use can be assessed, which allows identifying higher risk groups. Age of first use and frequency of use are included in the EMCDDA guidelines (European Model Questionnaire – EMQ). See this link for more information.
The concept of 'intensive users' has been often used. There is not yet a standard definition of 'intensive drug use' (or 'heavy drug use'). It is a broad term meaning use of the substance over a certain threshold of frequency or amount. It does not imply necessarily the existence of 'dependence/abuse' or other problems, but it will increase the risk of negative consequences, either dependence or others.
In this report, 'daily use' is used as an indicator of intensive use (in operative terms it is the use in 20 days or more in last 30 days). This frequency has been used in different studies, and can be derived from the EMQ. This threshold has been used for cannabis. Other substances (e.g. cocaine) may require different thresholds of frequency.
Although frequency measures are relatively simple and comparable, there is an increasing work in several countries to further assess the presence of problematic use or dependence through specific scales, as it has been done in other countries (USA or Australia). The EMCDDA is promoting collaboration among EU experts working in this area, with the aim to reach common methodologies.
The age ranges used to report results might have an influence in the results of prevalence estimates. Comparisons should be based on the same age groups. The EMCDDA recommends the age range 15–64 years for the whole adult population and 15 to 34 years for young adults. If wider age groups are used (e.g. 12 to 75 years) prevalence estimates will tend to be lower because illegal drug use is quite low at higher ages. If narrower groups are used (e.g. 18 to 49 years), estimates will tend to be higher because drug use concentrates among young adults.
Information provided by surveys is particularly useful when they are repeated at regular intervals, using the same questionnaires and methodology (a survey series), although this requires a long-term commitment from public institutions and research institutions.
Series allow tracking of trends over time that cannot be identified by a single survey or two consecutive surveys without further continuation.
Most Member States have conducted representative national surveys during recent years, although in some cases sample sizes are limited. On the other hand, several countries have conducted their first national surveys in the last years, in all cases with high compatibility with the EMQ.
There are still differences across countries in survey context, data collection methods and sampling procedures. In addition to methodological questions, several factors can contribute to differences in overall national figures. Relative proportion of urban and rural population in each country may explain in part some overall national figures. Also national figures may be explained in part by generational factors, including the different rates of convergence between the lifestyles of young males and females. Social context can influence also self-reporting of drug use. Comparative analysis across countries should be made with caution, in particular where differences are small, and formulation and evaluation of drugs policy should take carefully into consideration concrete age groups, birth cohorts, gender and urbanisation, among other criteria.
The EMCDDA has developed guidelines to improve comparability of population surveys in the EU. These guidelines include a set of common core items (EMQ) and basic methodological recommendations. The set of items can be used to report data from existing surveys, or can be inserted into broader questionnaires. The set includes basic prevalence measures and use patterns of certain illegal and legal substances, basic socio-demographic characteristics and opinion and risk perception questions. The questions about drug policies are considered optional. The guidelines have been compiled in an EMCDDA Handbook (see: http://www.emcdda.europa.eu/?nnodeid=1380).
The EMCDDA has also developed an EU Databank on Population Surveys on Drugs. This databank collates, on a voluntary basis, databases from existing national surveys already analysed and exploited at national level, in order to obtain an added value by further methodological and content understanding of drug patterns. The databases have been harmonised following the EMQ (ex-post harmonisation) (see: http://www.emcdda.europa.eu/?nnodeid=1380).