Although 942 students were recruited to participate in the pilot study, usable SE data were available for only 714 students (see Figure ). Students were excluded if they (a) provided no psycho-social palm pilot data or had missing/invalid ID numbers (n = 212), (b) had excessive missing data where they did not complete at least 70% of the items on any of the psycho-social scales (n = 16), or (c) did not complete at least 70% of the items within at least one of the FV or W SE questionnaires (n = 53). The final sample was nearly evenly split by gender (Table ). Approximately one-half of the sample was Hispanic (48.9%) and over one-fourth (27.3%) was Black. Few students (12.0%) had a college graduate head of household. Average student age was 11.3 years (± 0.6) and average BMI%tile was 70.7 (± 28.0).
Flow chart of participant recruitment and availability of complete and incomplete questionnaire and dietary consumption data.
The variables were first tested for missing completely at random (MCAR) using Little's likelihood-ratio test [32
]. Results indicated that the data were not MCAR (chi-square = 54.22, df = 37, p = 0.015). Bivariate chi-square tests of association between missing data status and demographic characteristics yielded a significant [X2
(3) = 8.76, p = 0.033] association only with race/ethnicity (see Table ). When MCAR was again tested, after excluding race/ethnicity, the results suggested that data were MCAR (chi-square = 31.32, df = 21, p = 0.068) when not considering race/ethnicity. The bivariate contingency coefficient (C = 0.10) showed this association was small. Hispanic [OR = 1.7 (1.1, 2.7)] and Other race/ethnicity participants [OR = 1.9 (1.0, 3.6)] were significantly more likely to have missing SE data. Because the chi-square is influenced by sample size and the difference was not meaningfully significant, MCAR was tested on 90% of the sample. After randomly selecting 90% of the 942 participants, the 90% of the sample demonstrated MCAR (chi-square = 50.86, df = 37, p = 0.064). Results suggest that the probability of responding to race/ethnicity (and other demographic information) was independent of responding to self-efficacy. As the significant association was more likely due to the sample size and less likely to depend on the strength of the association as evidenced by the contingency association and that the probability of responding to the demographic information was independent of responding to self-efficacy, the data were considered to be MCAR.
The largest sample available was used in each analysis. Listwise deletion, a conservative and less powerful, yet valid method for MCAR, was used where only the 664 students who provided at least some FV and W SE data were included in the psychometric evaluation. A large sub-sample of students (n = 625) who provided social desirability data and at least one measure of dietary consumption were included in the validation phase of the analyses.
The first factor accounted for 38.8% of the variance in the 22 F SE items with a second factor accounting for only an additional 7.5%, indicating, for the purposes of IRM, the scale was sufficiently unidimensional with a single major (or global) dimension. All F SE items had acceptable discriminability (corrected item total correlations) at 0.31 or higher. Cronbach's alpha was 0.84 across all items. IRM of the F SE scale revealed item difficulty estimates ranged from -0.94 (...sure that you can eat 1 portion of fruit for a snack at home at least one time) to 1.11 (...sure that you can eat 1 portion of fruit most times when you eat at a fast food place), and all items were within the fit criteria (Table ). Person separation reliability (comparable to Cronbach's alpha) was 0.82.
The first factor accounted for 47.1% of the variance in the 14 V SE items, with a second factor accounting for only an additional 9.5% of the variance, indicating a single major dimension scale. All the V SE items had acceptable discriminability at 0.39 or higher. Cronbach's alpha was 0.85 across all items. IRM of the V SE scale revealed that item difficulty estimates ranged from -0.79 (...sure that you can ask someone in your family to serve 2 vegetables for dinner at least one time) to 0.81 (...sure that you can eat 3 portions of vegetables at least 4 days a week, even when you are stressed), and all items met fit criteria (Table ). Person separation reliability was 0.83.
The first factor accounted for 48.7% of the variance in the seven W SE items (each item loading ≥ 0.51) with a second factor accounting for additional 20.3% of the variance, indicating acceptable unidimensionality. All the W SE items had acceptable discriminability at 0.28 or higher. Cronbach's alpha was 0.70 across all items. IRM of the W SE scale revealed that item difficulty estimates ranged from -0.78 (...sure that you can drink only water whenever you are thirsty for at least one day) to 0.74 (...sure you can drink 6 glasses or bottles of water at least one day), and all items were within fit criteria (Table ). Person separation reliability was 0.66, which is below acceptable standards (Table ).
The Wright maps (Figure ) revealed that the items in each scale covered only a restricted portion of the distribution covered by participants suggesting inadequate content validity, especially at the more difficult to respond end.
Wright map of fruit, vegetable, and water self-efficacy latent distribution and item difficulty estimates, with each "x" representing 4.5 cases.
The FV SE scales were highly intercorrelated (r = 0.72) and each was moderately correlated with W SE (F SE with W SE r = 0.50, V SE with W SE r = 0.44).
IRM analyses were repeated with the reduced sets of items (nfruit = 10 items; n vegetables = 8 items; n water = 5 items) with very similar results (not shown). The intra-class correlations between the full and reduced set of FV and W SE items were 0.95, 0.94, and 0.95 for F, V, and W SE scales, respectively (Table ).
Correlations among SE and Consumption variables, controlling for social desirability
Twenty-four hour dietary recalls (24hdr) were obtained on 432 of the children, with most (404, 93.6%) providing three or four days of recall. Single day intraclass correlations (ICC) were low for F (ICCF = 0.15), and V (ICCV = 0.16) intake, but modest for W (ICCW = 0.42) (Table ). Average (across the three days) ICC were modest for all three types of intake (ICCF = 0.35; ICCV = 0.37; ICCW = 0.68). Mean daily intakes were low with substantial variability for all three intake variables (Table ).
Sub-sample means, standard deviations for fruit, vegetable, and water self-efficacy and consumption
Internal consistency reliability on the FFQs were 0.83, 0.87, and 0.85 for F, V and W. The mean intakes from FFQ were substantially higher than from 24hdr (Table ). The FFQ scores were weakly correlated with social desirability, but the 24hdr estimates were not (Table ).
Only the V SE scale (abbreviated) was significantly correlated with social desirability. Correlations between the SE and intake variables corrected for social desirability, revealed both the long and abbreviated F SE scales were not significantly correlated with F intake. Both the long and abbreviated V SE scales were significantly, but weakly, correlated with V intake by both the FFQ and 24hdr estimates. Both the long and abbreviated W SE scales were significantly, but weakly, related to W intake as estimated by FFQ, but not by 24hdr.