This webpage explains frequently used statistical terms.
Count (or Numerator)
A ‘count’ is the number from a population with a condition of interest.
Counts are suppressed if it could lead to identifying individual persons.
Population (or Denominator)
A ‘population’ is the total number in a defined group.
A ‘mean’ is the average number (ie, adding all data points and dividing by the number of data points).
A ‘proportion’ is the count divided by the population.
Proportions are suppressed if the count<5 or population<30, due to unreliability of the estimate with small numbers.
A ‘percent’ is the proportion multiplied by 100.
Percents are suppressed if the count<5 or population<30, due to unreliability of the estimate with small numbers.
Crude Rate (CR)
A ‘crude rate’ represents the proportion of the population affected by a disease. It takes into account the population size. It does not take into account varying age distributions when comparing between populations.
Crude rates are suppressed if the count<5 or population<30, due to unreliability of the estimate with small numbers.
Age-Standardised Rate (ASR)
An ‘age-standardised rate’ is standardised to the World Health Organisation (WHO) population1 with a standard age distribution, to allow for comparisons between populations with differing age structures.
Age-standardised rates are suppressed if the overall count<20, due to unreliability of the estimate with small numbers, or if any age-band of the calculation has a population<30.
Confidence Interval (CI)
A confidence interval gives the range that the true value is likely to fall within. Generally, 95% confidence intervals are presented on Healthspace.
A small sample of the total population examined can result in a wide confidence interval between the lower level (LL) and upper level (UL).
For survey data, 95% confidence intervals give the range that if we selected 100 different samples, we would expect the prevalence value to fall within the range 95% of the time.
A ‘rate-ratio’ is a ratio of two rates.
Rate-ratios are suppressed if either of the rates are suppressed.
Statistics and methods
What is the difference between prioritised ethnic groups and total response ethnic groups?
Prioritised ethnic groups assign people who reported multiple ethnic groups to only one ethnic group, in the prioritised order: Māori, Pacific, Asian, European/Other.
Total response ethnic groups include everyone who identifies with that ethnic group. This means that the sum of the ethnic group populations will be greater than the total population.