Previous investigations of collective efficacy have lacked consistency in the way in which it has been conceptualised, operationalised, measured, and analysed. In addition; limited research has considered how collective efficacy might be manipulated to improve overall team performance. The broad aim of this thesis therefore, was to advance the understanding of collective efficacy measurement and its application in sport psychology. In chapter three, two separate studies were conducted to design and preliminarily validate a collective efficacy inventory for sport. Confirmatory factor analysis was used in the first study to assess the factorial validity of a pool of 18-items, and indicated that either a 10-item single-factor model or two 5-item models provided the closest fit to the conceptual model. In the second study, data collected using the 10 remaining items revealed both the 10-item and two five-item models had robust construct and criterion validity when correlated with three other theoretically related inventories. However, the two five-item models were highly correlated, indicating they measured the same construct. Therefore, given that longer inventories have greater internal reliability, the 10-item model was adopted as a measure of collective efficacy (Collective Efficacy Inventory; CEI) for the remainder of the thesis. The remaining experimental chapters of the thesis considered the psychological strategies appropriate for the manipulation of collective efficacy. Of the four basic psychological skills, imagery was proposed to have the strongest conceptual link with collective efficacy. Therefore, chapter four examined the relationship between different imagery types and individual perceptions of collective efficacy as a function of skill. Motivational general- mastery (MG-M) type imagery significantly predicted collective efficacy scores for the elite sample, indicating that MG-M type imagery was a suitable intervention for improving levels of collective efficacy. In chapter five, a multiple baseline across-groups design was then used to examine the effects of an MG-M type imagery intervention on perceptions of collective efficacy. Collective efficacy increased for the first group, became more consistent for the second, and did not change for the final group. Lower levels of intra-group variability were reported for all groups following the introduction of the intervention. The findings provided partial support for the use of MG-M type imagery interventions to enhance collective efficacy in an elite sports team. The overall findings of this thesis have increased understanding of the measurement of collective efficacy and its manipulation using imagery interventions. Practical recommendations are suggested for how the CEI can be used to monitor the effects of an imagery intervention on collective efficacy, and specific design implications for the delivery of the intervention to team sports
