Using the critical incident technique for qualitative process evaluation of interventions: The example of the “Let's Move It” trial

Rationale
Trials evaluating interventions to promote health behavior change rarely embed investigations that assess participant perceptions of crucial triggers of change.

Objective
The "Let's Move It” (LMI) randomized trial evaluated a theorybased whole school system intervention aiming to increase physical activity (PA) of adolescents attending vocational schools. This article serves two main purposes: to describe how to use the critical incident technique (CIT) to conduct in qualitative process evaluation to identify events, including intervention elements, which LMI trial participants perceived to enable or support behavior change.

Method
Semi-structured interviews (n = 34) conducted immediately post intervention from intervention and control arms were analyzed using the CIT.

Results
The analysis identified altogether 39 critical incidents. Most of the critical incidents were related to the LMI in the intervention arm and the findings are partly aligned with the LMI intervention theory. Analysis revealed several critical incidents also in the control arm, including gaining insights regarding PA and mere measurement effects, illustrating challenges facing real-world trials.

Conclusion
The CIT seems a promising approach for directing analysis towards potentially crucial intervention elements as described by the participants themselves, helping in focusing and limiting the text corpus to accounts relevant to change. Qualitative evaluations in trials may add valuable understanding to complement quantitative assessments.