Data issues

Rates

Incidence rates have been calculated as cases per million of the usually resident population of Australia. ABS population data were used for this purpose. Annual rates were calculated using finalised population estimates for each year. The 1998 finalised population estimate was not available at the time that this report was prepared and the 1997 estimate was used for that year. As the Australian population increases each year by about one percent, reported rates of SCI for 1998/99 could be about one percent higher than would otherwise be the case.

All-ages rates have been adjusted to overcome the effects of differences in the proportions of people at different ages (and different injury risks) in the populations that are compared. Direct standardisation was employed, taking the Australian population in 1991 as the standard.

All (or nearly all) cases of SCI are registered, so sampling errors do not apply to these data. However, the time periods used to group the cases (ie. calendar years) are arbitrary. Use of another period (eg. July to June) would result in different rates.

Confidence intervals

Where case numbers are small, the effect of chance variation on rates can be large. Confidence intervals (95%, based on a Poisson assumption about the number of cases in a time period) have been placed around rates as a guide to the size of this variation. Chance variation alone would be expected to lead to a rate outside the interval only once out of 20 occasions. An extreme rate in a single period of enumeration should not be ignored simply because of a wide confidence interval - a time series may show such a rate to be part of a trend.