Annex D Statistical definitions
Median
The median is the value above and below which half the cases fall, i.e. the 50th percentile, if the cases are sorted in ascending order. If there is an even number of cases, the median is the average of the two middle cases. The median is a measure of central tendency not sensitive to outlying values unlike the mean, which can be affected by a few extremely high or low values.
Percentage increases
The number of returns to the SDMD varys between years, both in terms of the total number of individuals and number of valid returns for specific data items. This results in a situation whereby the percentage of clients who report a particular activity or characteristic can rise, whilst the count of individuals within the category falls.
European Age Standardised Rate
Comparison of numbers of new individuals attending services may be misleading and, as such, a common adjustment made is for the effect of age. An age standardised rate is the incidence in the population that, theoretically, would be observed if the population had the same age structure as a standard population. This technique facilitates the comparison of attendance rates from differnet populations. In the Scottish Drug Misuse Database tables, the standard population used was the European Standard Population.
Confidence Interval
The 95% confidence interval of a rate shows the range of values within which we are 95% certain that the rate would lie if we had an infinitely sized population, and were basing the directly age-standardised rate upon that. For example, we are 95% confident that the rate lies between 166 and 219 per 100 000 population in the rate below, based on the data available.
Rate = 193 (166,219)
References
Advisory Council on the Misuse of Drugs (1982) Treatment and Rehabilitation, HMSO.
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