The Australian ERP as at 30 June 2001 is used as the standardising population throughout the report. Age‑standardisation of rates enables valid comparison across years and/or jurisdictions without being affected by differences in age distributions.

Population‑based rates of injury tend to have similar values from one year to the next. Exceptions to this can occur (for example, due to a mass‑casualty disaster), but are unusual in Australian injury data. Some year‑on‑year variation and short‑run fluctuations are to be expected, so small changes in a rate over a short period do not provide a firm basis for asserting that a trend is present.

All rate calculations utilise a denominator based on the estimated resident population (ERP) calculated as at the midpoint of each financial year. For example, for the reporting period 2021–22, the denominator population is the June 2021 ERP + the June 2022 ERP, divided by 2.  This is used as the denominator for age specific/crude and age standardised rates. Rates are calculated for each financial year unless otherwise noted.

Common measure calculations are outlined in Table 2.

Table 2: Common measure calculations

Population (used for rates) 

 June 21 population + June 2022 population2Numerator ÷   Denominator
Crude or age-specific rate of hospitalisation

Number of cases of injury hospitalisation per defined category (for example, age group)

Estimated Australian population as at mid-point of financial year(Numerator ÷ Denominator) x 100,000

Age-standardised rate (ASR).

Age-standardised rates were derived using 5-year age groups up to 85+. 

Age-standardised rates for First Nation populations were derived using 5-year age groups up to 65+.

Expected events per age group in standard population= crude rate of hospitalisation x standard population (for each corresponding age group) 

The direct method of standardisation is used.

(Sum of numerators across all age groups ÷ total standard population) x 100,000

Average length of stay

Number of patient bed daysNumber of cases Numerator ÷ Denominator, as days, rounded to 1 decimal place

Change in rates

 Sum of rate changes Number of yearsEstimated trends in age-standardised rates were reported as average annual percentage changes.
Rate ratiosCrude rate of hospitalisations for a defined age group among childrenCrude rate of hospitalisations among adultsNumerator ÷   Denominator
Crude rates of sports injuriesNumber of cases of injury hospitalisation per defined category (e.g. age group)AusPlay Participant estimate data for the given financial year      (Numerator ÷ Denominator) x 100,000

Note that ‘average length of stay’, as presented in this report, does not include some patient days potentially attributable to injury. It does not include days for most aspects of injury rehabilitation, which cannot be reliably assigned without information enabling identification of all admitted episodes associated with an injury case.

Due to rounding, percentages in tables may not add up to 100.0.