Rosenkranz et al. BMC Public Health 2010, 10:81 http://www.biomedcentral.com/1471-2458/10/81 Obesity prevention interventions are frequently deliv- ered in school settings, including a recent focus on after-school programs [37]. Some interventions have been implemented in community centers, churches, and youth clubs. Researchers have long recognized the importance of parents and the home environment on obesity prevention efforts, but the ability to bridge from institutional settings such as schools to parents and the home environment has been elusive for obesity prevention. Girl Scouts The Girl Scouts of the USA is a not-for-profit national organization that is a member of the world association of Girl Guides and Girl Scouts, and is dedicated to building the courage, confidence, and character of girls, to make the world a better place (http://www.girlscouts.org, accessed July 10, 2009). Girl Scouts may offer a viable channel for health promotion and obesity prevention interventions due to several inherent factors. First, the national organization is committed to promoting the health and well-being of girls, and several merit badges exist that reward girls for their efforts to improve knowl- edge and behavior related to PA, nutrition, and healthy living. Second, there is diversity amongst the members of Girl Scouts with regard to socio-economic status, race, and ethnicity. Third, the organization and troop leaders are focused on promoting youth development, and a sys- tem of socialization exists wherein the girls are expected to learn new skills, are empowered to make changes in their lives, and are asked to complete projects designed to demonstrate what they have learned. Several research- ers have used Girl Scout troops to deliver interventions designed to promote health behaviors [11,22,23]. Rationale for the present study Although a broad array of obesity prevention interventions currently exists, there are no published reports of rando- mized controlled trials targeting the promotion of health- ful scout meeting environments or family meals. There are an assortment of multi-level interventions based in schools and other institutions attempting to bridge health promo- tion and obesity prevention effects to the home environ- ment, but few of these interventions have explicitly attempted to impact the home environment by enhancing the skills of children within the institutional environment setting. Therefore, the purpose of this study was to evalu- ate an intervention designed to prevent obesity by modify- ing Girl Scout troop meeting environments, and by empowering girls to improve the quantity and/or quality of family meals in their home environments. Methods Study design This study is a group-randomized controlled trial using a nested cohort design, with troops being the unit of Page 3 of 13 randomization [38]. Our sample size and a priori power calculation were based on our research team’s analogous group-randomized trial taking place in after school pro- grams and powered to detect a 0.5 unit BMI change between 4 intervention and 4 control sites with 20 girls per site. Our post-hoc power calculations showed that the present group-randomized trial had 90% power to detect a primary outcome BMI-z score difference of 0.31 (roughly half the distance between the 85th and 95th percentiles) between intervention and control means, and 75% power to detect the same effect in our subgroup of overweight girls. Related post-hoc calcula- tions showed limited power to detect differences in indi- vidual outcomes of reported PA (23% power to detect a difference of 60 minutes per day, 1.5 days per week) and FV consumption (20% power to detect a difference of one serving per day). Seven troops agreeing to participate completed a pretest time 1 assessment within a two-week period in October before randomization. Troops were stratified into large (n = 4) and small size troops (n = 3) and then according to a random number generator were randomized (by first author) within strata to the con- trol or intervention conditions (see Figure 1). After being trained for implementation of the curriculum and supporting policies, the intervention troop leaders instituted the intervention components at the next scheduled troop meeting. A trained research assistant observed each troop during seven full meetings between time 1 (October, 2007) and time 2 (April, 2008) assessments to record troop meeting environ- mental variables, including leader health-promotion behaviors. At study commencement, research assistants were blind to condition of each troop. Following the seven observations, all troops underwent the time 2 assessment during a two-week period in April. The research protocol received approval from the IRB at Kansas State University (#4389). Sampling and Participants Girl Scout Troops To meet study inclusion criteria, the troop needed to be a registered Girl Scout Junior troop, consisting of girls in the 4th and 5th grades. To be registered, the troop leaders had to complete Girl Scout leader train- ing and pass a criminal background check. Troops also needed to meet at least twice per month, have meeting facilities capable of allowing physical activity and food preparation. Troops also needed to have initial agree- ment of leaders and parents for the troop to partici- pate in a research study. Exclusion criteria included troops not primarily composed of Girl Scout Juniors, not regularly meeting during the study period, or not having leader and parental consensus approval for