Disease Measurement

Epidemiology depends on precise quantification of disease in populations. Two fundamental concepts underpin disease measurement: incidence and prevalence. Understanding the distinction — and when to use each — is critical for interpreting public health data and designing studies.

Incidence measures new events over a defined time period. The incidence rate is calculated as the number of new cases divided by the person-time at risk. If 50 new cases of tuberculosis are diagnosed in a population of 10,000 over one year, the incidence rate is 50 per 10,000 person-years. Incidence proportion (also called attack rate in outbreak settings) is the probability of developing disease in a specified period — it has no unit of time in the denominator and is expressed as a proportion.

Prevalence measures existing cases at a point in time or over a period. Point prevalence is the proportion of the population with disease at a single moment. Period prevalence covers a defined interval (e.g., one month). The relationship between incidence (I), prevalence (P), and average disease duration (D) is summarised in the steady-state formula: P ≈ I × D. Conditions with long duration (e.g., diabetes) accumulate high prevalence relative to incidence; acute self-limiting conditions have similar prevalence and incidence.

The Standardised Mortality Ratio (SMR) allows comparison of mortality rates between populations with different age structures. The SMR is the ratio of observed deaths to expected deaths in the study population, multiplied by 100. An SMR of 150 means the study population has 50% more deaths than expected if they had the same age-specific mortality rates as the reference population. SMRs are used extensively in occupational epidemiology and in comparing Māori and non-Māori mortality in New Zealand.

Age-standardised rates (direct standardisation) apply the age-specific rates of the study population to a standard population's age distribution, allowing fair comparison. Crude rates ignore age structure — misleading when comparing populations with different demographic profiles. New Zealand uses the WHO World Standard Population for direct standardisation in NZMDS reports.

The burden of disease concept goes beyond mortality counts. Disability-Adjusted Life Years (DALYs) combine Years of Life Lost (YLL) — premature deaths weighted by expected life span — with Years Lived with Disability (YLD) — time spent in states of less than full health, weighted by disability severity. One DALY represents one year of healthy life lost. In New Zealand, the NZ Mortality and Disability Study (NZMDS) estimates DALYs for all major conditions. In 2016, musculoskeletal disorders, cancers, and cardiovascular disease were the top contributors to NZ health loss. Māori bear a disproportionately high DALY burden compared with non-Māori populations, reflecting both higher cause-specific incidence and greater disability.