1. Representativeness challenges for web surveys collecting information on drug users: a way for improving the cannabis market estimation in France.
Stéphane Legleye1, François Beck1, Stanislas Spilka1,2
1 CESP, Inserm, Villejuif, France
2 Observatoire français des drogues et toxicomanies, France

2. First French web survey among cannabis users
Methods and some descriptive results
Cannabis market estimation / limits
Comparison with other studies
Recommandations

3. The EMCDDA web survey
The EMCDDA developes estimates of the size of the drug market in the EU based on data provided routinely to the EMCDDA by National Focal Points
Problems:
The quantities of substances and expenditures of different groups of drug users may be known in detail through small scale surveys without representativeness
On the other hand, General Population Surveys generally do not have sufficiently large samples of drug users to provide robust information on amounts and expenditures
In 2016, a pilot study, in a first phase, was conducted by EMCDDA in different countries across Europe to overcome these difficulties:
Croatia, the Czech Republic, France, the Netherlands, Switzerland and the United Kingdom.

4. Recruitment: 4849 clics have been logged during the whole survey period : from 23 may to 1 september 2016 (101 days, i.e. 50 clics per day);
4849 clics on the web page
3738 accepted to participate (they had to give their agreement to participate)
2615 declared to live in France
2295 started at least one of the four modules relating their use drug habits
3000 used at least once one time last year cannabis or cocaïne or mdma or amphetamine (this was a necessary condition for participation)
1932 adults finally can be analysed : aged from 18 to 64 and with cannabis use in the past year

5. Strategy recruitment A dual strategy Through specialised websites
Links (banner) on 10 of the most important websites regarding drugs (self-support associations, prevention and information websites, etc.)
flyer ;
dedicated page on our web site;
twitter posts during the whole survey period;
The survey was relayed by several of our partners (university, research centres)
a banner on the site of a student health insurance (only 30 clics);
An advertisement posted on about pages Facebook pages: 3765 clics paid

6. A dual strategy
Après 5 semaine de la première phase de recrutement, on n’observe plus que 25 connexions. Dès le début de la campagne de recrutement via facebook le nombre de connexion remonte rapidement pour retomber dès l’arrêt de la campagne!

7. Web survey vs GPS: vs All 62 67 38 33 42 29 17 9 3 40 27 30 49 22 23%
WEB SURVEY 2016 (n=1932)
LAST GPS 2014
(n=1287)
Facebook (n=804)
Other (n=1128)
All
MALES 57 65 62 67
FEMALES 43 35 38 33
18-25 yo 78 60 42
26-34 yo 16 20 29
35-44 yo 4 13 17
45-54 yo 1 5 9
55-65 yo 2 3
SECONDARY SCHOOL 12
COLLEGE 48 40 27
UNIVERSITY 45 30
ALONE 52 47 49 22
COUPLE 25 23 15
COUPLE + children 6
ALONE + children 7
OTHERS 34

8. Web survey vs GPS: vs WEB SURVEY 2016 (n=1932) LAST GPS 2014 (n=1287)%
WEB SURVEY 2016 (n=1932)
LAST GPS 2014
(n=1287)
Facebook (n=804)
Others (n=1128)
All
PROFESSIONNAL STATUS
Missing 8 6 7 -
Employed 32 39 36 55
Student 51 40 45 20
Unemployed 11 9 17
Others 2 4 3

9. Frequency of use? Two sets of questions on web:
number of RESIN smoking days
number of WEED smoking days
Frequency based on the sum of the two
Frequency based on the maximum of the two
CANNABIS
SUM
MAX
GPS
Year
14%
41%
Month 1-9
23%
25%
30%
Month 10-19
11%
10%
Month 20-29
16% 4%
Month daily
37%
WEED
WEB
GPS
Year
18% ?
Month 1-9
36%
Month 10-19
15%
Month 20-29
16%
Month daily
GPS: only one question: « how many times did you smoke cannabis? »

10. Polydrug users and cannabis homegrowers in web respondents
Drug (past year)
Web
GPS
Cocaine
40% 8%
Amphetamins
33% 2%
MDMA
49% 6%
More cocaine and MDMA in non-facebook respondents:
43% vs 35% and 52% vs 46% respectively
Selection of cannabis homegrowers in web respondents
Weed frequency
Missing
Yes
Total
year
15% 4%
month1-9 6%
month10-19 3%
12%
month20-29 2%
17%
daily 1%
28%
Total (n) 97
224
1932

11. Quality of cannabis expenditure reports in web (resin and weed)
(Based on SUM)
N Obs
% Miss
Min
Mean
95th Pctl
Max
year
260
70% 30
300
920
month1-9
442
57% 26
100
600
month10-19
215
29% 42
110
360
month20-29
308
13% 86
250
460
daily
707
14%
189
550
4000
High level of expenditure missing, especially for low frequencies of use
Apparently abnormally high expenditure in low and high frequencies of use

12. Estimation procedure 2 types of data :
full data: observations WITH missing expenditures
Completed data: respondents WITHOUT missing expenditure
2 frequencies of use: based on SUM or MAX
Extreme values above p95 in each frequency of use recoded as non-response
2 estimation methods: IPW or matching
IPW: missing expenditures are imputed in web respondents with all variables available (homegrowing)
Matching: missing expenditures imputed after matching in GPS with GPS variables only (e.g. no homegrowing)

13. Some details (1)
IPW: propensity score: (variables present in GPS and web)
gender, age group, diploma, occupational situation, family situation, cocaine, amphetamines, MDMA
(All IPW-weighted web samples were well balanced compared to the GPS: standard differences less than 0,1)
IPW: imputation of missing expenditures in web:
Predictive mean matching
stratified by cannabis frequency of use
Gender, age, age-squared, diploma, family situation, occupational situation, homegrowing, cocaine, amphetamines, MDMA

14. Some details (2) Matching based on propensity score:
Propensity score used for IPW, stratified by frequency of use
Nearest neighbour with replacement: all GPS observations are matched
Matching: Imputation in GPS (after matching)
Predictive mean matching, stratified by frequency of use
Gender, age, age-squared, diploma, family situation, occupational situation, homegrowing, cocaine, amphetamines, MDMA

15. Some details (3) Finally we get 8 estimations of total expenditure
2 data x 2 frequencies x 2 methods = 8

16. Results (P95 troncated expenditures)
Frequency based on SUM
Frequency based on MAX
IPW
MATCHING
FULL
1,779 M€
2,216 M€
2,032 M€
2 399 M€
Complete
1 787 M€
1,904 M€
2,183 M€
2 212 M€
Matching yields higher estimates than IPW (due to frequent users)
No reliable mean to identify the best method
Average annual expenditure=2,064 M€, min=1,779 M€, max=2,399 M€

17. Summary of estimations
EXPERIMENTAL !
Annual expenditure for all cannabis products: 2,060 M€
Annual expenditure for weed only: 1,020 M€
Annual expenditure for resin only: 1,040 M€
Weed contributes to the half of the estimation

18. Validity Internal validity: some evidences
Small differences between methods
Matching and IPW led to balanced samples
BUT no frequency of weed smoking and homegrowing in GPS
External validity: uncertain !
Self-selection of frequent cannabis users that are polydrug users in the web survey
High expenditures in the web survey: drug dealers? Collective purchases? lack of precision in the web survey
Measurement mode effect (frequency of use, expenditures)?

19. Comparisons to previous estimations (1)
2005: M€: (Legleye et al. 2008)
2005 GPS, 2003 school survey (Espad) and adolescent survey (Escapad)
Expenditures in Escapad
number of joints on last occasion + method of supply
price*frequency in GPS
Partial disconnection between data sources
2016: 2,200 M€ [1,900; 3,100] (EMCDDA, 2016)
2014 GPS (same as used in this presentation)
frequency*price
arbitrary level of weed smoking frequency
Price estimated by retail and seizure data (external sources)
Disconnection between data sources

20. Comparison to previous estimations (2)
2010: 1,199 M€ (computations by the authors)
2010 GPS (n=1625 past-year cannabis users)
Report of frequency, expenditures, weed smoking and homegrowing
Single source for all data
Very strong internal and external validity
BUT: underreporting of expenditures?
Due to data collection mode effect (social desirability)?
Due to some lack of details: no distinction between weed and resin?
Recent changes between 2010 and 2014:
Number of past-year users raised by 29%
Rise of weed homegrowing: 6% (reported) to 15% (IPW)
evidence of a switch in the type of cannabis?
Weed is more expensive than resin (1.5 times)
Inflation of 5%

21. Recommandations for a replication
Try to recruit more diverse profiles
Ask comparable web and GPS questionnaires
On cannabis
Frequency of use assessed the same way (more simple on Web)
Frequency of herb smoking and homegrowing in GPS?
Personal or collective purchases on Web
On other drugs including alcohol and tobacco
On sociodemographics
Income
Rationale: adding control variables “reduces” self selection and measurement biases !
Ask expenditures in a GPS subsample to control for mode effect

22. References
Legleye S, Ben Lakhdar, Spilka S. (2008), Two ways of estimating the euro value of the illicit market for cannabis in France, Drug and alcohol Rev, 27(5),
Ben Lakhdar C, Lalam N, Weinberger D (2016), L’argent de la drogue en France, Estimation des marches illicites en France, Rapport à la Mildeca, 2016
EMCDDA (2016), Technical report, Estimating the size of the main illicit retail drug markets in Europe

23. ANNEX: expenditures for weed only

24. ESTIMATION for Weed ONLY
Problem: weed smoking days is not known in GPS
Wee keep the global frequency of cannabis use as stratification variable
Weed expenditures in the web survey
Weed frequency
N Obs
% Miss
Min
Mean
95th Pctl
Max
year
344
72% 9 50
300
month1-9
687
53% 21 70
400
month10-19
286
27% 45
150
month20-29
317
24% 87
210
500
daily
298
26%
161
1000
Some abnormally high expenditures
All expenditures above P95 will be recoded as non-response in each frequency category

25. Results (P95 troncated expenditures) for HERB only
Frequency based on SUM
Frequency based on MAX
IPW
MATCHING
FULL
939 M€
934 M€
1,074 M€
1,054 M€
Complete
929 M€
1,066 M€
1,103 M€
1,031 M€
Matching yields higher estimates than IPW
No real mean to identify the best method
Average annual total expenditure=1,016 M€, min=929 M€, max=1,103 M€