Changes in the incidence of assault after restrictions on late‐night alcohol sales in New Zealand: Evaluation of a natural experiment using hospitalisation and police data

AIMS To estimate the effect of national restrictions on late-night availability of alcohol on alcohol-related assault at a population level as indicated by (1) change in hospitalizations for weekend assaults and (2) change in the proportion of assaults documented by police that occur at night. DESIGN Evaluation of a natural experiment, involving: (1) pre-post comparisons of age-specific incidence rates, adjusted for seasonality and background trend using Poisson regression; and (2) interrupted time-series analyses, using seasonal autoregressive integrated moving average (SARIMA) models of national data with no control site. SETTING New Zealand. PARTICIPANTS (1) Inpatients discharged from NZ hospitals following assault during the weekend (Friday-Sunday) from 2004 to 2016 (n = 14 996) and (2) cases of assault recorded by NZ Police from 2012 to 2018. INTERVENTION introduction of national maximum trading hours for all on-licence (8 a.m.-4 a.m.) and off-licence premises (7 a.m.-11 p.m.), abolishing existing 24-hour licences, on 18 December 2013. MEASUREMENTS (1) Age-specific incidence of hospitalization for assault on Friday, Saturday or Sunday from the national hospital discharge data set, excluding short-stay emergency department admissions and (2) proportion of weekly police-documented assaults occurring between 9 p.m. and 5.59 a.m., from NZ Police Demand and Activity data set. FINDINGS Following the restrictions, weekend hospitalized assaults declined by 11% [incidence rate ratio (IRR) = 0.89; 95% confidence interval (CI) = 0.84, 0.94], with the greatest reduction among 15-29-year-olds (IRR = 0.82; 95% CI = 0.76, 0.89). There was an absolute reduction (step change) of 1.8% (95% CI = 0.2, 3.5%) in the proportion of police-documented assaults occurring at night, equivalent to 9.70 (95% CI = 0.10, 19.30) fewer night-time assaults per week, out of 207.4. CONCLUSIONS The 2013 implementation of national maximum trading hours for alcohol in NZ was followed by reductions in two complementary indicators of alcohol-related assault, consistent with beneficial effects of modest nation-wide restrictions on the late-night availability of alcohol.