Theory of Planned Behavior: Item Response Sets and Prediction of Physical Activity

Background: Less than half of Americans meet the recommendation of leisure-time physical activity (PA) of 150 minutes (CDC, 2012). A number of efficacious programs have been developed, and those that are based on theory are more effective. However, it is often difficult to determine the mechanisms of effect through meditational analyses. This is particularly an issue when a theory-based intervention is effective but theoretically hypothesized meditational relationships are not found. One reason for the lack of relationship could be the methods used to assess different theoretical constructs. The Theory of Planned Behavior (TPB) is one model used in the exercise and PA research domain which generally employs traditional fixed-graded measures of proposed theoretical mediators (e.g., strongly disagree to strongly agree response sets). More recent research provides initial evidence that using continuous-open scaling (e.g., ratio-level scaling; days or time/per week rather than agreement scales) has shown the superiority in measurement accuracy demonstrated by stronger relationships between the theoretical constructs and behavioral outcomes when compared to other scaling types. However, continuous open scaling has only been applied to correlational studies and there is no evidence that this scaling procedure results in measures that are sensitive to change or are related to both self-reported and objectively measured PA.

Primary Aims: The primary aims of this study were to determine 1) the sensitivity to change of TPB constructs using different response sets and self-report and accelerometer assessed PA, and 2) if TPB constructs measured using the different response-sets have differential prediction of PA measured using self-report and accelerometry.

Methods: Forty-six young adults were recruited to complete 13-item measure of TPB constructs using both fixed-graded and continuous-open scales as well as Godin's Leisure Time Exercise Questionnaire at 3 time (T1=Baseline, T2=End of week 1, T3=End of week 2) points over 2-weeks. Potential participants were excluded if they engaged in PA of 150 minutes or more per week. Inclusion criteria were the ability to perform moderate intensity PA and aged 18-25 years old. The order of different scales for the TPB constructs was randomly and evenly assigned within each condition. Participants were asked to wear an accelerometer for 2 weeks; one week prior to the action planning activity and one week after. To determine the sensitivity to change of the measures, participants were randomly assigned to either complete a personal action plan for physical activity (AP) or not (Control). Due to the exploratory nature of the pilot we set the significance level for all tests at p<0.10.

Results: In general responses to the continuous open versus fixed closed items, at each time point, resulted in significantly (p<.05) lower perceptions of attitude (instrumental attitudes T1=4.4, T2=4.2, T3=4.3 versus time1= 6.2, T2=6.4, T3=6.3; affective attitudes T1=3.6, T2=3.5, T3=3.9 versus time1= 5.0, T2=5.0, T3=4.8), subjective norm ( T1=3.4, T2=3.3, T3=2.4 versus time1= 5.3, T2=5.2, T3=5.3), perceptions of control ( T1=3.2, T2=3.7, T3=3.9 versus time1= 4.6 T2=4.9, T3=5.2), and intention (T1=1.5, T2=1.8, T3=2.4 versus time1= 4.8, T2=5.1, T3=5.2). In regards to sensitivity to change continuous open and fixed closed measures of instrumental attitudes, subjective norms did not demonstrate significant changes as a result of action planning.

Affective attitudes measured by the continuous-open scale, but not when measured by the fixed-closed scale, showed change over time regardless of condition. Perceived behavioral control measured using the continuous-open scale increased for AP participants by approximately 0.5 compared to control participants change of approximately 0.1 (p=.09). A similar pattern was found with intention in that changes in the continuous-open scale were significant (AP=0.9; control=0.2; p=0.07). No other scales showed significant sensitivity to change. Self-reported PA increased significantly for AP participants (81-16 minutes per week of PA) when compared to control participants (87 +/- 19 to 75 +/- 17 minutes per week of PA; p<0.1). Same pattern of differences was shown between AP (65 +/- 13 to 107 +/- 15 minutes per week of PA) and control (70 +/- 14 to 65 +/- 16 minutes per week of PA) participants (p<0.05).

Conclusions: Continuous open scaling have significant correlations with all constructs along with affective attitude and intention being correlated with the actual reported exercise behavior over fixed graded scaling. This data sheds further insight into the different response sets of the TPB in application to exercise domain within a sedentary, young population. The lack of a significant difference may be due to the small sample size. Further research should investigate the role of the personalized action plan utilizing a larger sample size and the correlation of the TPB with intention and actual exercise behavior within an intervention.