Page 81 - ON THE WAY TO HEALTHIER SCHOOL CANTEENS - Irma Evenhuis

pre- and post-intervention survey were subtracted and categorized into the dichotomous variable indicating a healthy or unhealthy change in purchase behaviour. A healthy score was defined as (1) a higher increase in healthier products compared with less healthy products; (2) a higher decrease in less healthy products compared with healthier products; or (3) purchases remained stable over time and consisted mainly of healthier products. An unhealthy score was defined as (1) a higher increase in less healthy products compared with healthier products; (2) a higher decrease in healthier products compared with less healthy products; (3) purchases remained stable over time and consisted mainly of less healthy products or an equal number of healthier and less healthy products.
Other student variables
Demographic student variables included age (in years), gender and current school level (vocational (i.e., VMBO), senior general education (i.e., HAVO) or pre-university education
(i.e., VWO)). Determinants of purchase behaviour included attitudes, subjective norms, perceived behavioural control and intention, all towards buying healthier products at
school. For each variable, multiple questions (range 2–5) were asked on a 5-point Likert
scale (answers ranging from, e.g., 1 = very unlikely to 5 = very likely) derived from existing
validated Dutch questionnaires [103, 104]. The mean score of each variable was calculated
and the reliability of the measurements was assessed with Cronbach’s alpha [186]. The 5 measured environmental determinants were having breakfast (Yes, No); amount of money
spent on food/drink purchases at school per week (<€1, €1–2, ≥€2); external food/drink purchase behaviour (<1 times p/w, 1–3 times p/w, ≥4 times p/w); and foods/drinks brought from home (<4 times p/w, ≥4 times p/w).
Sample size
The sample size was calculated based on the outcome purchase behaviour, an expected 10% drop out, 80% power and 5% significance level [130]. The calculation showed that 20 schools and 100 students per school were necessary to be able to detect a 10% difference in purchase behaviour of students (continuous variable), with the expected multi-level structure (students within schools, intra-class correlation of 0.05).
Statistical analyses
Student baseline characteristics and pre- and post-intervention canteen outcomes and student purchase behaviour were described by means and standard deviations. Canteen outcomes included three subtopics of the health level of the canteen: healthier food and drinks available in the cafeteria, in the vending machines and accessibility of healthier food and drinks. Mean (SD) pre- and post-intervention values and mean changes were described and changes in the subtopics per school were presented in a chart.
A mixed logistic regression analysis [110] was performed to investigate the effect of the intervention (independent variable) on purchase behaviour (dependent variable). Correlated errors of student scores (level 1) nested within schools (level 2) were taken into account by including a random intercept for schools in all analyses (model 1). The analyses were stratified by gender, as boys seems to react more to environmental changes than girls [187]. Models were first extended with demographic variables (model 2), secondly with students’ behavioural determinants (model 3) and thirdly with students’ environmental determinants (model 4).
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