Sample size calculation

The two main outcomes of interest in this study were (a) frequency of hospital presentation and (b) proportion of presentations with severe self-harm out of all hospital presentations for self-harm. To address (a) we simply collected presentations from as many different areas as possible within the available data collection period. To address (b) we used evidence from one London emergency department obtained in March 2020 that suggested that while the number of presentations might be reduced to a quarter the number in 2019, within those with presentations for self-harm the proportion of severe self-harm might have increased from around 8% in 2019 to around 40% in 2020. Based on a two-sided Fisher’s exact test with significance level 5% and a power requirement of 90% we calculated that a minimum of 84 self-harm presentations in 2019 and 21 self-harm presentations in 2020 would be required to detect such an effect within a hospital area.