Table 1.

Description of mediating variables

Measure	Source	Questions and psychometric properties
Psychosocial variables
Self-efficacy	Dishman et al. 2002 Motl et al. 2000	Self-efficacy for physical activity was measured by asking girls to indicate on a 5-  point Likert scale how well they agreed with eight statements such as “I can be  physically active during my free time on most days” and “I can ask my parent or  other adult to do physically active things with me.” The eight statements were  anchored from disagree a lot to agree a lot and were originally developed for 8th  and 9th grade girls. In the TAAG study, the Cronbach alpha ranged from 0.81 to  0.84 in the 2003, 2005, and 2006 surveys; the test-retest correlation was 0.67 at  6th grade and 0.69 at 8th grade
Outcome expectation	Dishman et al. 2002 Dishman et al. 2005 Motl et al. 2000	Outcome expectation was measured by asking girls to indicate on a 5-point Likert  scale how well they would agree with nine statements such as “If I were to be  physically active during my free time on most days...it would help me spend  more time with my friends” or “...it would put me in a better mood” or “... I  would make new friends.” The nine statements were anchored from disagree a lot  to agree a lot. In the TAAG study, the Cronbach Alpha ranged from 0.82 to 0.84  in the 2003, 2005, 2006 surveys; the test retest correlation was 0.64 at 6th grade  and 0.68 at 8th grade
Outcome expectancy	Dishman et al. 2002 Dishman et al. 2005 Motl et al. 2000	Outcome expectancy value was measured by asking girls to indicate on a 5-point  Likert scale “How important are these things?” Girls responded for 9 items such  as “spending more time with my friends is...” “Controlling my weight is...”,  “making new friends is...”. Each item was anchored from very unimportant to  very important. The Cronbach alpha ranged from 0.92 to 0.94 in the 2003, 2004,  and 2005 surveys; the test-retest correlation was 0.58 at 6th and 8th grade
Behavioral variables
School sport program	Developed for TAAG	Participation in school sports programs was measured by asking girls “which sports  teams have you been on during the past year at school?” with 15 options and scores  ranging from 0 to 15. Participation in out-of-school sports programs was measured by  asking girls “which sports teams have you been on during the past year outside of  school?” with 15 options and scores ranging from 0 to 15. These two items were  analyzed individually, as sum scores, with values ranging from 0 to 15
Activity participation	Developed for TAAG	Participation in after school physical activity programs was measured by asking  girls to answer yes or no to the question “Did you participate in any programs that  were physically active (like lessons, clubs, or sports teams) after school this school  year?” Participation in before school physical activity programs was measured by  asking girls to answer yes or no to the question “Did you participate in any  programs that were physically active (like lessons, clubs, or sports teams) before  school or during lunch this school year?” These questions were included only on  the 2005 and 2006 surveys, not on the 2003 survey. These items were analyzed as  individual 0/1 items
Environmental variables
After school transport	Evenson et al. 2006	Difficulty getting from a school activity was measured by asking girls to indicate on  a 4-point Likert scale anchored from not at all difficult to impossible “if you  stayed after school for an activity every day, how difficult would it be for you to  get home afterwards?” Difficulty getting to a community activity was measured  by asking girls to indicate on the same scale “If you wanted to do an after school  activity someplace else besides school every day, how difficult would it be to get  there?” Difficulty getting from a community activity was measured by asking girls  to indicate on the same scale “If you wanted to do an after school activity  someplace else besides school every day, how difficult would it be for you to get  home afterwards?” For the three items, test–retest reliability among 480 girls,  indicated by weighted kappa coefficients, ranged from 0.38 to 0.41. These items  were analyzed individually, so we have no scale properties to report, nor do we  have test-retest correlations to report
Access to facilities	Evenson et al. 2006	Access to facilities was measured by asking “Is it easy to get to and from this place  from home or school?” Girls responded for 14 places such as basketball court,  health club, park, or tennis court. Scores were calculated by adding the number of  items, ranging from 0 to 14. The Cronbach alpha ranged from 0.80 to 0.81 in the  2003, 2005, and 2006 surveys. This measure was originally developed and piloted  among 480 girls in 6th and 8th grade, with test-retest reliability of 0.78 (95% CI  0.74–0.84), indicated by an intraclass correlation coefficient
Providing social   support	Developed for TAAG	Providing social support to others for physical activity was measured using a single  item by asking, “During a typical week, how often do you encourage your friends  to do physical activities or play sports?” The five response options ranged from  never to everyday
Friend social support	Sallis et al. [51]	Friends’ social support for physical activity was measured using three items from  the Amherst Health and Activity Study[51]. The questions include: “During a  typical week, (1) how often do your friends encourage you to do physical  activities or play sports? (2) Do physical activities or sports with you? (3) Tell you  that you are doing well at physical activities or sports?” Test-retest reliability for  the peer scale was 0.86 and the Cronbach Alpha for those three items ranged from  0.74 to 0.79 in the 2003, 2005, and 2006 surveys
Family social support	Sallis et al. [51]	Family social support for physical activity was measured using five items modified  from the Amherst Health and Activity Study with acceptable measurement  properties. The questions include: “During a typical week, how often has a  member of your household...(for example, your father, mother, brother, sister,  grandparent or other relative): (1) encouraged you to do physical activities or play  sports? (2) Done a physical activity with you?; (3) provided transportation to a  place where you can do physical activities or play sports? (4) Watched you  participate in physical activities or sports? (5) Told you that you are doing well in  physical activities or sports?” Overall social support for physical activity was  computed by summing the three social support scores