Belief importance and the theory of planned behaviour: comparing modal and ranked modal beliefs in predicting attendance at breast screening.

OBJECTIVE: The objective of this study was, to compare the predictive utility of two measures of the attitude, subjective norm and perceived behavioural control constructs of the theory of planned behaviour (TPB; Ajzen, 1991) in predicting intention and subsequent attendance at breast screening. One construct was based on a modal set of underlying beliefs; the other was based on the three beliefs from each construct considered by the participant to be the most important.
METHOD: We used a prospective, longitudinal design using a postal questionnaire at Time 1 and objective attendance data from screening records at Time 2. Questionnaires were sent to 1657 women from southeast England due to be invited for X-ray mammography under the UK's National Health Service Breast Screening Programme. After evaluating a set of modal behavioural, normative and control beliefs, women were asked to select the three beliefs they saw as the most important for them, and to rank them. The products of these three beliefs formed the 'important' (vs. the modal) measures.
RESULTS: The 'important' attitude and subjective norm measures showed similar associations with direct measures and were equivalent to the modal measures in predicting intention and attendance at screening. The modal control construct was marginally more robust than the 'important' one in its association with the direct measure of control, and with intention. Key individual important beliefs that predicted intention and behaviour were identified.
CONCLUSIONS: Measuring belief importance can help more fully identify the structures underlying attitude, subjective norm and perceived behavioural control, and can provide useful information when the TPB is used as the basis for intervention to help change behaviour.