The Well London program - a cluster randomized trial of community engagement for improving health behaviors and mental wellbeing: baseline survey results

Table 8 Updated sample size calculations for the adolescent outcomes using the between-cluster coefficient of variation from the baseline survey (with missing responses multiply imputed)

Outcome	Baseline prevalence or mean (across all intervention and control clusters)	Km	Minimum detectable effect size	Expected effect size in original study design
Adolescents
Healthy eating	56% eat fruit daily or almost daily	0.003	17% increase in prevalence	30% increase in prevalence
Unhealthy eating score^b	Mean unhealthy eating score = 3.0	0.06	Decrease of 0.26 standard deviations
Physical activity^c	Mean PAQ-A score = 2.7	0.04	Increase of 0.25 standard deviations	-
Mental health and wellbeing	Mean PANAS-positive score = 29.8 (ranges 11 = lowest positive affect, to 55 = highest positive affect)	0.03	Increase of 0.23 standard deviations	-
	Mean PANAS-negative score = 18.0 (ranges 11 = lowest negative affect, to 55 = highest negative affect)	0.02	Decrease of 0.21 standard deviations
	Mean SDQ = 12.4 (range 0–15 = normal, 16-19 = borderline, 20- 40 abnormal)	0.03	Decrease of 0.22 standard deviations	30% increase achieving key thresholds for mental health
	74% Have normal SDQ scores	0.12	31% increase in prevalence
	29% have borderline or abnormal SDQ scores	0.04	14% decrease in prevalence

^aEffect sizes for binary outcomes are relative increases in prevalence.
^aCalculations are based on multiply imputed datasets; comparison to complete cases showed no substantial differences except in K_m for adolescent fruit consumption which decreased from 0.05 to 0.004.
^bPossible range 1 to 5; higher score indicates more frequent consumption of unhealthy food items (chips, chocolate or sweets, and sugar sweetened beverages).
^cRange 1 to 5; 1, very inactive; 5, very active.

ISSN: 1745-6215