Darcy Ummels

32 | Chapter 2 Strength of the relation and systematic difference The correlation between the number of steps measured by the activity trackers and the gold standard was weak for all activity trackers ranging from r =–0.02 for the Moves to r =–0.33 for the Digi ‐ Walker CW ‐ 700 (Table 2.5). The average underestimation of all trackers and the average overestimation of the Activ8 revealed a significant systematic difference with the gold standard for step count, expect for the Fitbit One ( P =0.35). Table 2.5 Correlation coefficient of the activity trackers and the gold standard for step count. Activity tracker Correlation coefficient ( P value) t value ( P value) Accupedo 0.32 (0.02) –9.4 (<0.001) Activ8 0.24 (0.06) –3.9 (0.001) Digi ‐ Walker CW ‐ 700 –0.33 (0.02) –6.2 (<0.001) Flex 0.31 (0.04) –5.1 (<0.001) Lumoback 0.19 (0.20) –6.2 (<0.001) Moves –0.02 (0.88) –3.4 (0.001) One –0.15 (0.30) –0.9 (0.35) UP24 0.09 (0.52) –6.9 (<0.001) Walking Style X 0.25 (0.08) –12.3 (<0.001) Level of agreement In all plots the limits of agreement are high, with the highest limits of agreement (–471.3 to 721.0) for the Activ8 (Table 2.4). In the plots, 2 trends are visible: either an over ‐ and underestimation of the number of steps during the activity protocols as shown in Figures 2.4 and 2.5 (e.g., Fitbit One and Activ8) or an underestimation of the number of steps only, as shown in Figure 2.6 (e.g., Digi ‐ Walker CW ‐ 700). Depending on the height of step count, overestimation or underestimation was shown. Overestimation became more pronounced when participant took more steps and vice versa. Systematic difference between short and long protocols Only the Walking Style X, Accupedo, and Fitbit Flex were used in both protocols. For all trackers, there were no systematic differences found for the average mean difference in step coun between the short and long protocols.