Changes in the incidence of assault after restrictions on late-night alcohol sales in New Zealand: evaluation of a natural experiment using hospitalization and police data

Abstract

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.