Methods and definitions

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 measure directly drug use, patterns of use, and related factors (both potential determinants and consequences of use of drugs) for each individual under study. A number of factors can be investigated retrospectively, although with the limitations that self-report and memory biases have on recall of past events.

When necessary, precision of estimates can be improved by increasing sample sizes, for instance when policy evaluation requires reliable estimations broken down by gender, age groups or regions, or when it is necessary to increase the reliability of estimates for substances with low prevalence rates. On the other hand, it should be considered 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).

In addition to the increase of sample size, certain sampling strategies may help to improve estimations among groups of particular interest; for instance oversampling of young people, ethnic minorities, or inner city areas.

The EMCDDA has developed guidelines to improve comparability of population surveys in the EU. These guidelines include a set of common core items (‘European model questionnaire: 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.eu.int/?nnodeid=1380).

The EMCDDA has also developed a 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.eu.int/?nnodeid=1380).

Surveys provide estimates of the proportion of the population who have used different drugs during certain periods of time. For illegal drugs, the more 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, and it is often used in policy debates. Lifetime experience alone may not capture well the current drug situation, as it also includes all those who have ever tried drugs. On the other hand, it can give a rough estimation of the extent of drug experience in the population, which is valuable for those drugs of lower prevalence. In addition, its analysis by age group (or birth cohort) can give insight into the generational dynamics of drug use; when a particular drug became popular. It is necessary for estimation of incidence (year of first use among ever users), and for computation of continuation and discontinuation rates (and eventual related factors) among those who have used drugs.

‘Recent use’ produces lower figures, but reflects better the present situation, giving an indication of recent but probably also some occasional use. 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 (sometimes last 30 days users are also considered as ‘regular users’), and will include the more intensive forms of use. The figures are generally low when the whole adult population (15 to 64 years old) is considered, except for cannabis.

However, 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 old) 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 (EMQ).

Intensity of use can be estimated through frequency scales; for instance, number of days of use in a given period of time (last 12 months or last 30 days). In the EMQ, measuring the number of days of use in the last 30 days assesses the intensity of use.

The concept of ‘intensive users’ has been often used, although using different scales of what ‘intensive use’ means. Many experts use this term as equivalent of ‘daily or almost daily users’ (use more than 20 times in the last 30 days), at least in the case of cannabis. This concept of ‘intensive use’ was used in the selected issue on cannabis of the 2004 Annual Report, and proved it was feasible and useful.

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 range 15 to 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) 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), which allow tracking of trends over time that cannot be identified by a single survey or two consecutive surveys without further continuation. This requires a long-term commitment from public institutions and research institutions.

Most Member States have conducted representative national surveys during recent years, although in some cases sample sizes are too small or the compatibility with the EMQ limited. On the other hand, several countries have conducted recently their first national surveys, in all cases with high compatibility with the EMQ.

Most of the new Member States or candidate countries have conducted recently national population surveys, with high compatibility with the EMQ.

Several countries have established series of national surveys or are starting them (Germany, Greece, Spain, France, Netherlands, Slovakia, Sweden, the United Kingdom and Norway). Finland has several consecutive surveys with relatively comparable methodology. But only few countries have consolidated series, with enough sample sizes, and in general only for a limited number of years.

There are 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 proportions 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.