The questions about illicit drug use2were all comprised within a single page in the ques-
tionnaire, and belonged to the core part of the latter concerned with the measurement of deviant behaviours. After a selective question about whether the subject has ever used illicit drug in his/her life, other information where gathered about the age of onset, the frequency and prevalence of use in the last twelve months since the last measurement, and the typologies of substance taken. These information are reported below.
1For more details see the translated excerpt of the questionnaire in the Appendix. 2This question does not include alcohol use, which is measured in a separate question.
Age of onset
The following table reports the average age of onset for the 5 time points. This variable refers to the use of all types of illicit drugs.
Table 4.1: Age of onset for drug use
2002
2003
2004
2005
2006
Mean
12.20
12.93
13.61
14.19
14.68
s.d.
1.40
1.22
1.22
1.25
1.41
Mode
12
13
14
14
15
Minimum
14
15
17
18
19
Maximum
6
9
9
8
8
The increasing value is a consequence of the growing number of subjects that each year start consuming illicit substances. An approximate average age of onset can be calculated as the mean across the above reported values. This average age of onset for drug use is 13,5. Another way to summarize the data presented in the table is to take the last response as reference value for all five time points. In the ideal situation a subject, who started using drugs prior to the first measurement, should be able to report the same value of this variable at all five waves. In many cases, however, there are discrepancies in the reported age of onset; some respondents were not able to replicate the same response at the successive measurements. An explanation could be that with the passing of time some subjects were not able anymore to recollect facts that had happened many years before. Consequently this problem could also indicate a poor reliability of the given information. In any case, the utility and interpretability of this variable is strongly reduced.
Life-time prevalence of drug use
Table 4.2 reports the proportion of subjects in the sample that used drugs at least once in their life. This variable refers to the use of all types of illicit drugs.
Table 4.2: Life-time prevalence of drug use
2002
2003
2004
2005
2006
%
6.1
12.9
19.4
24.9
27
At the first time point in 2002, when the subjects were on average 13 years old, only about 6% of the sample had already had experience with drugs. In the course of the study, across 5 waves, the proportion of subjects who admit having used drugs increases constantly up to about 27% at the last measurement point in 2006, when the subjects are about 17 years of age. It can be concluded, that by the end of adolescence, about one third of the interviewed youths have tried, at least once, an illicit substance.
4.2. Drug use
Last year prevalence of drug use
Table 4.3 reports the proportion of subjects who admited having used illicit drugs in the last 12 months. This variable refers to the use of all types of illicit drugs.
Table 4.3: Last year prevalence of drug use
2002
2003
2004
2005
2006
%
5.4
12
17
19.1
17.3
There is an increasing percentage of respondents who report using drugs more reg- ularly over time. This pattern reaches a peak in 2005 at the age of 16 with 19% of the sample using prohibited substances and decreases thereafter.
Although the life-time prevalence increases constantly over the years, it is not the same for the last-year prevalence; the latter decreases at the last time point. These results suggest that although the number of those who have used at least once illicit substances increases, in many cases it is a mere experimental behavior, which is later abandoned.
Frequencies of drug use
Table 4.4 reports the frequencies of drug use for the five measurements. This items measure how often a subject consumed illicit substances in the last year.
Table 4.4: Frequencies of drug use
2002
2003
2004
2005
2006
mean
.35
1.51
4.50
4.93
5.79
s.d.
2.65
12.40
26.67
30.98
32.63
skewness
12.20
15.44
9.15
9.57
7.87
kurtosis
189.48
276.20
98.07
102.94
71.30
max.
50
250
365
365
365
min.
0
0
0
0
0
The values’ range increases constantly from a maximum of 50 times in a year to a maximum of once a day. The distribution of the data in all five measurements is highly left skewed, consequence of the large number of zeros among the respondents (i.e., those who did not consume). Similarly, the large kurtosis reflects the presence of possible “outliers”, or people who reported a high frequency of use (see the maximum value: 365, once a day). Such a high consumption is, however, plausible, and these subjects should not be excluded from the analysis.
Although the proportion of subjects who use illicit drugs decreases in the last wave (see Table 4.3), the frequency of use increases constantly over time. This might well reflect the changing status of drug use among growing adolescents; whereas at the beginning of the
adolescence drug use is probably mainly an experimental incident (most of the subjects consumed only once, thus the low mean frequency), with growing age those who continue might do it more often (see the average use at age 17: nearly 6 times).
Typologies of drugs
Table 4.5 reports the percentages of subjects in the whole sample that used one of the men- tioned illegal substances in the last twelve months. Being a separate question and allowing the respondent to give multiple answers, in many cases it is not possible to associate this variable to the others mentioned in the sections above. In other words, when a multiple answer is given (e.g., one person reports the use of marijuana and heroin/morphine), it is not possible to discern whether the frequencies and all other questions refer to one of the given substances.
There are seven different drug items, which are labelled as follows: • t0295, Cannabis (Hashish/Marijuana) • t0296, Heroin/Morphine • t0297, Cocaine/Crack • t0298, Speed • t0299, Ecstasy/Designer Drugs • t0300, LSD/Mushrooms • t0301, other drugs
In the following table the prevalence of use for the single groups of drugs is reported for each time point.
Table 4.5: Prevalence of use of different drugs in percent (from 2002 to 2006)
%
t0295
t0296
t0297
t0298
t0299
t0300
t0301
2002
3.2
0.4
0.4
0.6
0.5
0.5
2.2
2003
10.2
1.2
1.6
1.7
2.3
1.6
3.5
2004
16.2
0.8
1.2
1.9
2.1
1.7
3.1
2005
18.9
0.8
1.6
2.6
2.5
1.8
2.4
2006
17.1
0.5
1.7
2.2
2.3
1.5
2.1
In general, over the whole time span the most consumed drug is cannabis and its derivates. Only a small portion of the sample reports the use of any other substance than marijuana. Worth to be noticed is the large portion of subjects who responded, especially at the beginning of the study, to the category “other drugs”. The great variety of names used in everyday language to define a specific type of drug might have led to great confusion, especially among the younger consumers, who might have not found the corresponding name in the questionnaire.
Taking the year 2005 as an example, where the larger proportion of the sample used illicit substances, it can be noticed that nearly 19% reported cannabis consume. The second most used substances are, thereafter, party-drugs like speed, ecstasy, and designer drugs