Page last updated: 25-MAR-2008

General Practice - Practice Team Information (PTI)

PTI statistical notes

The occurrence of many health conditions varies by factors such as age, gender and/or the relative level of deprivation of populations. The PTI sample population is broadly representative of Scotland in each of these factors. Nonetheless, to adjust for small differences in age, gender and deprivation between the PTI sample and the total Scottish population, the method of direct standardisation is applied in producing all the estimates published here. Normally standardisation would use a 'standard' population (as defined in epidemiological literature), whereas in this case standardisation is carried out using the Scottish population (which does not have a pre-defined, even distribution over age and gender groups). Rates are calculated within each sub-class (i.e. within all age-gender-deprivation categories) and these rates are then multiplied by the number of people within the corresponding sub-class in Scotland. The resulting estimated numbers are then aggregated to the desired level. For example, to show figures by gender, the subcategory estimates would be summed over all age and deprivation categories within each gender. To then show rates, the estimated (gender-specific) numbers are divided by the total number of people in Scotland within each gender. Hence the level of standardisation will depend on the aggregation:

  • For information presented by age and gender, rates are standardised by deprivation.
  • For information presented by gender (all ages), rates are standardised by age and deprivation. As the age distribution in the reference population (the current Scottish population) is different in men and women, differences in (all-ages) rates between men and women may be partly due to differences in age.
  • For information presented at a "total" level (over all age, gender and deprivation categories), rates are standardised by age, gender and deprivation.

The measure of deprivation used in standardisation of PTI analysis published here is the Scottish Index of Multiple Deprivation (SIMD).

Work is currently underway within the PTI analytical team to further improve our statistical methods, in particular to develop a sound method of calculating confidence intervals for the estimates provided, which will in turn involve revisiting the detail behind the standardisation process. A major issue to address is the commonly made assumption that patients recorded within PTI are independently sampled from the Scottish population, whereas in fact practices are sampled (not patients). This leads to a rather different error structure, and the impact of this is currently under investigation.

More detail on the statistical methods is available on request from the PTI team.