Partes de la clase Características

While the intermediate variables are not as central to this thesis as the parental influences, they are important in terms of intermediate confounding and as mediators. As such their measurement and relationship to the outcome is discussed here in some detail. The ALSPAC data offered very good coverage of the intermediates: of the 22 concepts posited in the conceptual I-DAG only three were not measured (beliefs, economic activity, and extended family effects). Two more were only measured before the exposure (adolescent BMI and sibling effects) and were thus reassigned as baseline confounders. In terms of the quality of the measurements used, intra-personal and interpersonal effects had much better matches than the institutional effects of school and neighbourhood. Table 7-6 below summarises pertinent information on the intermediate variables, including exposure status, proportion exposed, age at measurement, sample size, and unadjusted associations with the outcome. Each of the non-parental intermediates that were measured had harmful unadjusted associations with AUDIT at 16.5 years of age, except for IQ which had a protective unadjusted association. The below discusses measurement of the intermediate variables very briefly. Box 7-2 below briefly summarises intermediate variables for ease of reference.

139 Box 7-2: Intermediate variables reminder

7.3.5.1 Intra-personal effects

The conceptual I-DAG identified the following intrapersonal intermediates: attitudes to alcohol; early alcohol initiation; smoking; other substance behaviours; risk-personality; general personality; beliefs; general behaviour; mental health; physical health; and economic activity. Most were measured adequately in ALSPAC. Attitudes to alcohol were measured at age 14 by asking the adolescent how ‘harmful or helpful’ alcohol consumption is to physical and mental wellbeing. Early alcohol initiation was measured at age 12.5 by asking the adolescent how old they were when they first had a whole drink of alcohol. Smoking was measured at age 13.5 by asking the adolescent if they had ever smoked. Other substance behaviour was measured at age 14 by asking adolescents if they had ever tried any of a comprehensive list of illicit substances. Risk personality was measured using the Arnett’s sensation-seeking scale at age 13.5, which produced a score of sensation seeking tendency. General personality was measured using IQ at age 15.5. This measure was the closest match to general personality, as other questions tended to overlap strongly with mental health (e.g. psychosis). General behaviour was measured as anti-social behaviour at age 14 by asking if the adolescent had ever engaged in one of several anti-social behaviours (theft, vandalism, verbal assault, etc.). Mental health was measured using the Short Mood and Feelings questionnaire at age 13.5 years, giving a ‘score’ for current depressive symptoms. Physical health was measured using BMI and pubertal timing. However, BMI was measured before maternal drinking and so was reassigned as a

confounder. Pubertal timing was measured by assessing the adolescent’s development of body hair at As noted in Chapter 2 and Chapter 3, intermediate variables have specific profiles defined by the directed edges between them and the exposure, other intermediates, and the outcome. For any intermediate variable, directed edges must either be received from the exposure, or not exist between it and the exposure. A variable which ‘sends’ a directed edge to the exposure is not an intermediate but a confounder. Intermediates which do not receive an edge from the exposure are mediator-outcome confounders. Intermediates which do receive an edge from the exposure are defined in terms of how they relate to the mediator of interest. If we want to estimate the effect through an intermediate which receives a directed edge from the exposure, then it is of course the mediator. If an intermediate which receives a directed edge from the exposure in turn sends a directed edge to the mediator, then it is an exposure-induced

mediator-outcome confounder (EIMOC). If it receives a directed edge from the mediator and the exposure then should not be factored in the model at all. These are sometimes referred to as ‘nuisance mediators’ (Dunn et al., 2015).

140 age 13. The only intra-personal intermediate concepts that were not measured before the outcome at age of 16.5 were economic activity and beliefs.

7.3.5.2 Interpersonal effects

Interpersonal intermediates suggested by the conceptual I-DAG were: number of friends; peer drinking; peer smoking; peer substance use; peer anti-social behaviour; sibling effects; relationships with teachers; and extended family influences. No information on the latter was available in the ALSPAC data. Number of friends was measured at age 13.5 by asking the adolescent how many ‘close friends’ they had. Peer drinking and peer smoking were also measured at age 13.5 by asking if the adolescent’s friends had smoked tobacco or drank alcohol without parental permission. Peer substance use was asked at age 14 years by asking if the adolescent’s friends had tried any of the same list of illicit substances asked of the adolescent as above. Similarly, peer anti-social behaviour was measured in the same way as the adolescent’s own, by asking if the adolescent’s friends had engaged in one of several anti-social behaviours. Sibling effects could only be measured by ‘number of older siblings’ at age 7, which was reassigned as a confounder. Relationships with teachers were measured by asking how much the adolescent agreed that their teachers were “OK” at age 16.5. As such, it cannot be used as an EIMOC for any of the other mediators, as it is measured after them.

7.3.5.3 Institutional effects

School engagement was measured by asking the adolescent at age 14 how strongly they agreed on a Likert scale with the statement that school is a “place where they really like to go each day”. Academic performance was measured using number of GCSEs between A* and C (taken in year 12 when pupils are aged between 15 and 16). Neighbourhood attachment was measured by asking the adolescent how safe they usually felt in their neighbourhood. The only one of these three variables that is not problematic is school enjoyment. Number of GCSEs is an ideal fit for academic

performance conceptually, but the timing of its measurement was after all parental exposures. Similar to relationships with teachers, this meant it could not be treated as an EIMOC, but rather only as a potential mediator. Neighbourhood safety does not offer adequate coverage of community

engagement. Nonetheless, both of these variables were retained. No non-parental family effects were used as intermediates – sibling effects, family structure, family transitions, and family history of alcohol were all assigned as confounders.

141 Table 7-6: Summary of intermediate measures

Concept Variable Exposure Age N %

Exposed

Unadjusted OR (95% C.I.)

Intra-personal

Early alcohol Early alcohol initiation Before age 10 12.5 3,765 29.1 1.31 (1.09, 1.58)

Risk personality Sensation seeking Upper quartile on Arnett’s sensation seeking scale

13.5 6,045 30.0 2.23 (1.76, 2.37)

Mental health Depression Score from Short Mood and Feeling

questionnaire, 1= >8

13.5 6,078 28.7 1.40 (1.20, 1.62)

Attitudes towards alcohol

Positive alcohol attitudes Does not think regular alcohol consumption or binge drinking are “very harmful”

14 6,089 30.1 1.68 (1.50, 1.93)

General behaviour Anti-social behaviour Has engaged in anti-social behaviour (e.g. theft, vandalism)

14 6,178 36.3 2.40 (2.10, 2.74)

Smoking Smoking Has smoked 14 5,908 25.9 2.94 (2.53, 3.42)

Substance use Substance use Has tried an illicit substance 14 5,911 20.2 2.51 (2.14, 2.94)

General personality IQ +1SD on IQ score 15.5 4,370 19.8 0.77 (0.65, 0.92)

Other interpersonal

Number of friends Number of close friends 7 or more 13.5 6,108 22.8 1.33 (1.13, 1.56)

Peer smoking Peer smokers Friends have smoked cigarettes without parental permission

13.5 5,297 55.1 2.44 (2.12, 2.81)

Peer drinking Peer drinkers Friends have drunk alcohol without parental permission

142 Peer anti-social

behaviour

Peer anti-social behaviour Friends have engaged in anti-social behaviour (e.g. theft, vandalism)

15.5 5,357 45.5 3.04 (2.64, 3.51)

Peer substance use Peer substance use Friends have tried an illicit substance 15.5 5,348 53.9 3.21 (2.77, 3.73) Relationships with

teachers

Relationships with teachers Does not “strongly agree” that most of their teachers are “OK”

16.5 5,284 63.2 1.65 (1.43, 1.90)

Institutional Neighbourhood

attachment

Neighbourhood safety Adolescent usually doesn’t feel safe in their neighbourhood

14 5,449 6.7 1.55 (1.20, 2.01)

School engagement School engagement Adolescent does not like attending school 14 5,552 35.2 1.24 (1.08, 1.43) Academic performance Academic performance >8 GCSEs grade A*-C 16 12,064 62.4 1.23 (1.08, 1.40)

Reassigned as confounders

BMI BMI BMI at age 11 11

143