# Sample size requirements and intra-cluster correlations for stepped wedge cluster randomised trials in intensive care medicine: A practical guide

## TL;DR

This paper provides practical guidance for planning stepped wedge trials in ICUs using real-world data from Australian and New Zealand hospitals.

## Contribution

The study offers region-specific intra-cluster correlation and cluster auto-correlation estimates for ICU trials.

## Key findings

- Observed mortality ranged from 10.3% to 23.0% across different ICU subgroups.
- ICCs ranged from 0.008 to 0.022 and CACs from 0.83 to 1.00.
- A 50-ICU trial could achieve 81.6% power to detect a 2.7% mortality reduction.

## Abstract

To estimate key statistical parameters and provide practical guidance for planning stepped wedge cluster randomised trials in Australian and New Zealand intensive care units (ICUs).

Cross-sectional retrospective observational study using routinely collected ICU data.

Adult public hospital ICUs contributing to the Australian and New Zealand Intensive Care Society Adult Patient Database between 2010 and 2023.

All adult ICU admissions to 132 ICUs. Subgroups included unplanned admissions and admissions involving invasive mechanical ventilation or vasopressor use.

In-hospital mortality during the index hospitalisation within 90 days of ICU admission. Intra-cluster correlation coefficients (ICCs) and cluster auto-correlations (CACs) were estimated using exchangeable, block-exchangeable, and discrete time decay models using a cross-sectional design.

Among 1,291,849 eligible ICU admissions, observed mortality ranged from 10.3% (all ICU admissions) to 23.0% (non-elective invasively ventilated patients in Mega-ROX ICUs). ICCs ranged from 0.008 to 0.022 and CACs from 0.83 to 1.00, with block-exchangeable or discrete time decay models most often providing the best fit. In a worked example, a 50-ICU stepped wedge trial with 10 steps (11 two-month periods) enrolling 45 unplanned ventilated patients per ICU per period (total ≈24,750 patients) would have 81.6% power to detect an absolute mortality reduction of 2.7%.

Stepped wedge cluster randomised trials are feasible for evaluating ICU-wide interventions when routine data are available. The ICC and CAC estimates presented here provide Australian and New Zealand-specific parameters for future trial planning and demonstrate the potential of this design for pragmatic large-scale ICU research.