Yost et al. BMC Public Health 2010, 10:465 http://www.biomedcentral.com/1471-2458/10/465 Demographic data for the current study were obtained from the Add Health Wave I in-home interviews (Stage 2), which followed the same data-collection procedures as described for Wave II. Although further information is available from the Add Health Web site http://www. cpc.unc.edu/projects/addhealth, Add Health-sponsored publications lack information regarding the validity and reliability of the study’s in-home interviews. In addition, recommended techniques were not available in the cur- rent secondary analysis study [32]. Measures Demographic Variables Each subject’s age was calculated by subtracting the sub- ject’s date of birth from the date of data collection recorded on the laptop computer used for data collec- tion. Subjects ethnicity was identified as Hispanic or Latino or not Hispanic or Latino. Subjects also self-iden- tified themselves into one of the following race cate- gories: White, Black or African American, American Indian or Native American, or Asian or Pacific Islander. For the current study, subjects’ socioeconomic status was operationalized to include two separate measures. Research on health-disparities lacks accepted measures of socioeconomic status [33-35]. Although single and composite measures exist, evidence suggests that occu- pation may not be a useful indicator of adolescents’ SES and that educational attainment, which is not inter- changeable with family income, is the most widely used indicator of adolescents’ SES [33,35]. Therefore, in the current study, the two operationalized measures for SES were highest maternal educational attainment and family income [1994 household total income (in thousands of dollars) before-tax and household income received in 1994 (including income from all household members, dividends, welfare benefits, and other sources)]. Body-Mass Index The measure used for actual weight, BMI, was calcu- lated as weight (in kilograms) divided by height (in meters) [kg/m2]. Although standardized classification systems of overweight and obesity for adolescents do not exist, classifying adolescents’ weight based on age- and gender-specific BMI cut-off points is accepted worldwide [36]. In the present study, classifications from the National Health and Nutrition Examination Survey were used to identify female adolescents’ BMI for age and gender as obese (> 95th percentile), overweight (85th to 95th percentile), or normal weight (< 85th percentile). Dichotomous variables were created to identify subjects as obese or overweight. Weight Perception As an important perceptual dimension of body image, weight perception is defined as the perception of one’s body weight [37]. In the Add Health Wave II in-home Page 4 of 11 interviews, subjects’ weight perception was assessed by the question, “How do you think of yourself in terms of weight?” Response choices included “very underweight,” “slightly underweight,” “about the right weight,” “slightly overweight,” or “very overweight.” The responses of “very underweight,” “slightly underweight,” and “about the right weight” were collapsed into a single “not over- weight” response of weight perception. Accuracy of Weight Perception To measure the accuracy of subjects’ weight perception, each subject’s BMI and weight perception were com- pared. From this comparison, four variables were cre- ated. The first variable (reference category) indicated subjects who were not overweight or obese and per- ceived themselves in any of the weight perception cate- gories (very underweight, slightly underweight, about the right weight, slightly overweight, or very over- weight). The second variable indicated overweight or obese subjects who underestimated their weight. Despite being overweight or obese, these subjects did not per- ceive themselves as overweight (weight perception was very underweight, slightly underweight, or about the right weight (Overweight or Obese, Not Overweight weight perception). The third and fourth variables indi- cated subjects with a degree of accuracy in their weight perception. These subjects were overweight or obese and perceived themselves as either slightly overweight (Overweight or Obese, Slightly Overweight weight percep- tion) or very overweight (Overweight or Obese, Very Overweight weight perception) (Table 1). Due to the small number of female subjects who were obese and perceived themselves very overweight (n = 88), it was not possible to construct a separate weight perception accuracy variable for subjects whose BMI percentile indicated they were obese. Trying to Lose Weight To determine whether subjects were currently trying to lose weight, responses to the question, “Are you trying to lose weight, gain weight, or stay the same weight?” were examined. To create a variable that identified sub- jects who were currently trying to lose weight, the responses “gain weight” and “stay the same weight” were collapsed together into “not trying to lose weight.” Engaging in Exercise as a Weight-Loss Method An aim of this study was to determine whether BMI and weight perception are associated with trying to lose Table 1 Classification of Weight Perception Accuracy Weight Perception BMI Classification Overweight or Obese Not Overweight Inaccurate Underestimation Slightly overweight Accurate Estimation Very overweight Accurate Estimation