The STrAtegies for RelaTives (START) intervention is a psychological therapy to reduce anxiety and depression in family carers of people with dementia living in the community. It was developed and evaluated, using a randomised controlled trial, by a multidisciplinary team including essential contributions from statisticians. This was the first UK trial of a manual-based therapy for dementia carers. The trial randomised 260 carers to START or usual care and assessed them for anxiety and depression over 6 years. Statistical analysis required methods not widely used in applied research to address specific trial complexities, ensuring unbiased and precise effect estimates. Linear mixed models accounted for clustering of outcomes by therapist and joint models for informative truncation of repeated measurements at care-home admission or death. Analyses showed START was clinically and cost-effective in the short- and long-term. The trial impacted UK policy and guidance, being highlighted in the Prime Minister’s Challenge on Dementia 2020 and in the National Institute of Clinical Excellence guidelines on Dementia (2018). In 2020 START was being used by at least 24 services across 12 NHS Trusts and by charities including the Alzheimer’s Society. It has also been translated and adapted for use in several other countries.