Regression equations

Regression equations were developed for the prediction of follow up PSW from time in moderate (Y = .206 + .618 (baseline PSW) + .035 (Baseline time in moderate); time in vigorous (Y = .618 + .657(baseline PSW) + .020 (baseline time in vigorous); and time in MVPA (Y = .265 + .635 (baseline PSW) + .017 (Baseline time in MVPA) for the ≥10 hours for ≥ 4 days criteria.

For the ≥8 hour for ≥4 days criteria, the regression equations were as follows.

For time in moderate, Y = .387 + .685 (baseline PSW) + .025 (baseline time in moderate), and for time in MVPA, Y = .393 + .699(baseline PSW) + .013 (baseline time in MVPA).

R²∆ SEE B t p

>10 hours for >4 days

Time in Moderate .144 .409 .035 2.873 .007 Time in Vigorous .109 .423 .020 2.416 .022 Time in MVPA .164 .401 .017 3.120 .004

>8 hours for >4 days

Time in Moderate .058 .408 .025 2.299 .026 Time in MVPA .064 .406 .013 2.412 .020

157 5.4. Discussion

In this chapter children’s compliance with various wear time combinations of days per week and hours per day were examined. In addition, the effect of applying two commonly used wear time standards on values of exposure and outcome measures, as well as other covariates, was examined at three different time points along with cross-sectional correlations between measures. Finally, the effect of wear time on the longitudinal association between baseline PA and PWB was examined. The main findings show that compliance decreases with stricter wear time criteria for both days and hours per day. Further, compliance reduces with repeated measures across time points, regardless of the criteria used, though stricter criteria resulted in even lower compliance levels. Selection bias was apparent with the use of both wear time criteria at baseline and mid-point measurement, while misclassification of time in PA intensities was apparent across all time points in all intensities. Importantly, longitudinal relationships between PA and psychological variables differed depending upon the wear time criteria employed.

5.4.1. Compliance

As was expected, the percentage of children complying with increasing hours per day and increasing days per week reduced substantially at each time point.

Across time, compliance with at least 8 hours or 10 hours wear per day for the minimum of three weekdays and one weekend day also decreased with each subsequent time point. At baseline 75% and 50% of children complied with the 8 and 10 hours wear times respectively and by the third time point, compliance had decreased to 36.9% and 27.4%. In summary, children’s wear-time decreased over multiple time points, with stricter wear-time criteria resulting in lower levels of compliance. Not only did compliance decrease but the average weekly wear time decreased substantially, with a drop in accrued wear time of approximately 10 hours for each subsequent measurement point.

Though a decrease in compliance was expected with subsequent time points, the extent to which non-compliance occurred at all-time points was greater than anticipated. Van Covering and colleagues (2005) reported that full wear-time (7

158 days; 13 hours for weekdays and 9 hours at weekends) was acquired from 50%

of participants at a singular time point, yet in the present study, neither the 8 nor 10 hour criteria resulted in compliance as high as this, even at baseline.

Furthermore, studies reporting reductions in compliance across time points, have not reported such dramatic decreases as those apparent in the present study. Fitzgibbon et al. (2011) reported post intervention compliance of 65.9%, while Audrey et al. (2012) reported slightly lower compliance of 56%, much higher rates than seen at post – intervention for the present study (≥10 hours = 27.4%; ≥ 8 hours = 36.9%, this may be due to the additional time point included in the present study. Compliance rates at mid-point (≥10 hours = 35.6%; ≥ 8 hours = 49.3%) are closer to those reported in earlier studies with follow up time points. Interestingly, Audrey and colleagues (2011) required more wear-time per day for fewer days (10 hours for ≥ 3 days) compared to the Fitzgibbon study (8 hours ≥ 4 days), yet reported lower levels of compliance. It may be reasonable to suggest then that more lenient wear criteria of hours rather than days would results in higher proportions of participants included in a study. Results in table 5.1 support this idea. Further, a higher proportion of children achieved Fitzgibbon’s criteria than Audrey’s criteria at each time point.

The results of the present study are in agreement with those reported by Colley et al. (2010), who showed that participant numbers increased with more lenient wear-time criteria; the authors reported that 79.5% of participants recorded >10 hours wear time and 83% of participants recorded >8 hours wear time, however it was unclear as to the number of days of data, that these values were in respect to. Colley et al. (2010) also reported the number of valid days achieved by children over one week’s measurement period; showing that 86%

of males and 88% of females aged 6 – 11 years achieved four days of monitoring for a minimum of 10 hours per day. The present study reports lower compliance rates with this at all time points, but extends the findings of Colley and colleagues by reporting the proportions of children complying with multiple hour and day criteria. This allows for comparison of compliance with other studies who employ different wear time criteria, such as those reported by Page et al. (2010), who employed a wear time criteria of ≥ 3 day for ≥ 10hours and saw compliance rates of around 77%; similar to the portion of children meeting that criteria in the present study at baseline. Similarly King et al. (2011) used a

159 criterion of ≥ 3 day for ≥ 6hours, and reported compliance of 94.7%, again similar to those in this study. The results reported in this chapter, along with those reported by others suggest that children find it easier to comply with shorter hours for more days rather than stricter hours for fewer days.