General population surveys — an overview of the methods and definitions used
Prevalence and patterns of drug use in the general population (adults or school children) is assessed through surveys based on representative probabilistic samples of the whole population under study. Interview surveys are based on self-report of participants regarding present and past behaviours, personal characteristics, knowledge and attitudes regarding drugs or other health topics.
Surveys allow a direct estimation of prevalence of drug use, as a proportion of the population that declare having used specific drugs. Also, they facilitate estimating patterns of use and other factors considered as potential determinants or consequences of drug use for each individual under study. The factors that are investigated retrospectively have the limitations of self-report (concealment) and memory biases on recall of past events.
The prevalence estimates of surveys are based on standard periods of time. For illegal drugs, the more usual measures are:
- any use during the person’s life (or lifetime prevalence), also called ‘lifetime experience’,
- any use during the previous year (or last 12 months prevalence), also called ‘recent use’,
- any use during the previous month (or last 30 days prevalence), also called ‘current use’.
‘Lifetime experience’ alone will not capture the current drug situation among adults (although it is considered useful among school children) as it also includes people that tried drugs a long time ago. On the other hand, it is a framework measure; it can give a first rough estimation of the extent of drug experience for low prevalence drugs, and can help to estimate patterns of use such as incidence, length of drug use, or continuation or discontinuation of use, including eventually characteristics and the reasons of those who quit.
Last year prevalence (‘recent use’) produces lower figures, but better reflects the present situation, although often use could be occasional. Combination of lifetime experience and recent use can give basic information on drug use patterns (e.g. as discontinuation of use — people that have used in the past, have not used in the last year — or continuation of use — those that have used in the past, have also used in the last year — when incidence of use is taken into account, although after a certain age, incidence of drug use is very low).
Last month prevalence (‘current use’) gives some indication of more regular use and will include the more intensive users, although in fact many current users will not be intensive users.
Estimates of ‘recent’ or ‘current’ for the whole population (15–64 years) are usually low in many countries, except in the case of cannabis. It makes more sense from an epidemiological and policy perspective to focus the analysis on young people (15–24 or 15–34 years) particularly among males, where drug use concentrates. Additionally, analysis should consider urban versus rural areas and other lifestyle and social characteristics.
Samples sizes should enable specific analysis with a minimum level of precision, and it is not recommended to use too small samples. In particular, when policy evaluation requires reliable estimations broken down by gender, age groups or regions, or when is necessary to increase the reliability of estimates for substances with low prevalence. In addition to the increase of sample size, certain sampling strategies may help to improve estimations among groups of particular interest; for instance over-sampling of young people, ethnic minorities, or inner city areas.
It should be acknowledged that there are limitations of surveys in estimating prevalence of the more marginalised forms of drug use (e.g. heroin injection, crack use) due to the low prevalence figures, but in particular due to not probabilistic errors (exclusion from the sampling frame, absence of household, non-response). On the other hand, surveys identify a small but not negligible number of people that had used heroin in the past, but not at present, and they could be useful to examine characteristics of these users, to gain insight in eventual reasons to explain their not continuation of use, compared to those long-term users identified in treatment centres.
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).
The concept of ‘intensive users’ is used sometimes, often in relation to cannabis use. There is as yet no universally accepted definition of ‘intensive drug use’. It is, however, a broad term meaning use of a drug (e.g. cannabis) that exceeds a certain threshold of frequency. It does not necessarily imply the existence of ‘dependence/abuse’ or other problems, but it is considered to increase the possibility of negative consequences, including dependence.
In this report, ‘daily use’ is used as an indicator of intensive use of cannabis (in operative terms it is the use in 20 days or more, in the last 30 days). This frequency has been used in different studies, and can be derived from the European Model Questionnaire (EMQ). For other substances (e.g. cocaine) it may be necessary to define different thresholds of frequency to consider the use ‘intensive’.
Although frequency measures are relatively simple and comparable, there is ongoing work in several countries and at the EMCDDA, to further assess the presence of problematic use or dependence through specific brief scales included in surveys (or eventually in other settings such as telephone or internet helplines). Also, similar work is being developed in other countries (
In the case of adults, but even more so in the case of children, the age ranges used to report results might have an influence in the results of prevalence estimates, and comparisons should be based on the same age groups. For adults, the EMCDDA recommends range 15–64 years for the whole adult population and 15–34 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. For children, see below.
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.
For adults, 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 drug surveys in the last years, in all cases with high compatibility with the EMQ.
Information on trends in prevalence of use, and on the characteristics of users are particularly valuable. Several countries have established series of national surveys, or have started them in recent years. For the time being, only a limited number of countries have longer term series with big sample sizes. As these series continue and new ones become consolidated, the possibility of interpretation of trends will increase. This will help to plan and evaluate policies.
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. The 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.
In the case of adult surveys, 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.
Regarding surveys among school children data are derived mainly from The European School Survey Project on Alcohol and Other Drugs (ESPAD) and the WHO Health Behaviour in School-aged Children Survey (HBSC). Participation in both surveys, each conducted every four years,has grown in each round and includes both EU Member States and non-EU countries with over 20 EU Member States participating in the most recent surveys, together with Norway and Bulgaria, Croatia, Romania and Turkey.In addition, annual, or biannual, national schools surveys are conducted in
See the section on ‘Methodology of school surveys’ in the Statistical bulletin.
Finally, the EMCDDA has also developed a prototype of 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 the prototype database from Germany, Greece, Spain and the United Kingdom). The aim was to demonstrate the feasibility and added value of further analysis at EU level, after national analysis and use of the data has been completed (a period of about three to five years). The national databases were harmonised following the European Model Questionnaire (ex-post harmonisation), including only the common set of core items (See here). In the medium term it will be considered additional methods to allow collaborative analysis among groups of countries that are willing to do that and EMCDDA (e.g. workshops).