Background: The minimal clinically important difference (MCID) is relevant in the estimation of improvement in a patient outcome.Aim: To determine the MCID on the Aberrant Behaviour Checklist-Irritability (ABC-I), widely used to measure the effects of intervention for aggressive challenging behaviour in people with intellectual disabilities.Method and procedures: We utilised distribution and anchor based methods to estimate the ABC-I MCID. We extracted data from 15 randomised controlled trials (RCTs) for meta-analysis. We conducted three online workshops with family carers and professionals to consider meaningful change in case vignettes of increasing severity of aggressive challenging behaviour.Outcomes and results: We did not find overlap in the range of values between the two approaches. The meta-analysis indicated a range of MCID on the ABC-I (0.05, 4.94) whilst the anchor-based estimation indicated a larger change (6.6, 16.6).Conclusions and implications: The MCID is essential in interpreting the results from intervention studies. The present work was undertaken as part of a wider programme on the development and testing of a psychosocial intervention for aggressive challenging behaviour, and it is of interest to researchers in justifying how they choose and determine the MCID on the outcome of interest.