Active (Opt-In) Consent Underestimates Mean BMI-z and the Prevalence of Overweight and Obesity Compared to Passive (Opt-Out) Consent. Evidence from the Healthy Together Victoria and Childhood Obesity Study.

Claudia Strugnell,Boyd Swinburn,Liliana Orellana,Lynne Millar,Joshua Hayward,Steven Allender

doi:10.3390/ijerph15040747

Abstract

Background: Tracking population trends in childhood obesity and identifying target areas for prevention requires accurate prevalence data. This study quantified the magnitude of non-participation bias for mean Body Mass Index-z scores and overweight/obesity prevalence associated with low (opt-in) compared to high (opt-out) participation consent methodologies. Methods: Data arose from all Local Government Areas (LGAs) participating in the Healthy Together Victoria Childhood Obesity Study, Australia. Primary schools were randomly selected in 2013 and 2014 and all Grades 4 and 6 students (aged approx. 9–12 years) were invited to participate via opt-in consent (2013) and opt-out consent (2014). For the opt-in wave N = 38 schools (recruitment rate (RR) 24.3%) and N = 856 students participated (RR 36.3%). For the opt-out wave N = 47 schools (RR 32%) and N = 2557 students participated (RR 86.4%). Outcomes: differences between opt-in and opt-out sample estimates (bias) for mean BMI-z, prevalence of overweight/obesity and obesity (alone). Standardized bias (Std bias) estimates defined as bias/standard error are reported for BMI-z. Results: The results demonstrate strong evidence of non-participation bias for mean BMI-z overall (Std bias = −4.5, p < 0.0001) and for girls (Std bias = −5.4, p < 0.0001), but not for boys (Std bias = −1.1, p = 0.15). The opt-in strategy underestimated the overall population prevalence of overweight/obesity and obesity by −5.4 and −4.5 percentage points respectively (p < 0.001 for both). Significant underestimation was seen in girls, but not for boys. Conclusions: Opt-in consent underestimated prevalence of childhood obesity, particularly in girls. Prevalence, monitoring and community intervention studies on childhood obesity should move to opt-out consent processes for better scientific outcomes.

Highlights

Stemming the obesity epidemic is one of the greatest public health challenge for 21st century [1]with approximately 23% [2] of children and adolescents in developed countries overweight or obese
School participation rates were similar in both study waves (24.4% vs. 32.0%; NS), while student participation rate was 36.3% under opt-in and 84.4% under opt-out consent (p < 0.001) (Table 1)
The mean Body Mass Index z-score (BMI-z) score, estimated under opt-in strategy was “outside” the range of means obtained when resampling from the opt-out sample, that is, lower than the mean calculated for any of the 10,000 samples

Summary

Introduction

Stemming the obesity epidemic is one of the greatest public health challenge for 21st century [1]with approximately 23% [2] of children and adolescents in developed countries overweight or obese. In high-income countries such as Australia, 20% of youths aged 5–17 years were classified as overweight and a further 7% as obese in 2014/15 [3]. These children and adolescents are at risk of the myriad of acute and chronic conditions associated with obesity [4], which typically persist into adulthood [5]. Most studies among school-children require opt-in (active consent) from parents or guardians and achieve participation rates (PRs) between 30–60% [9]. Prevalence, monitoring and community intervention studies on childhood obesity should move to opt-out consent processes for better scientific outcomes

Methods

Results

Conclusion