Methods of quantifying disease Stuart Harris Public Health Intelligence Analyst Course – Day 3

2 Learning objectives Methods of quantifying disease To understand the definitions of, and differences between, the following methods: Absolute counts Ratios Proportions/percentages Rates Incidence and prevalence

3Methods of quantifying disease Quantitative Measures of Health Status Measures of health status convey information about the occurrence of death or disease. They include: Counts Proportions Ratios Rates

Counts 4Methods of quantifying disease Simplest/most frequently performed measure in epidemiology Refers to the number of cases of an event or characteristic e.g. number of people who died from lung cancer in England in 2012 May be useful for allocation of health resources or planning of health services e.g. number of people who had hip replacements in Bristol in 2013/14

Absolute count example 5Methods of quantifying disease Area A: 348 people have asthma Area B: 972 people have asthma Is there a difference in frequency of disease between area A and area B? Cannot answer without more info. e.g. Area A: population = 3,500 Area B: population = 20,000

Proportion Definition: a part, share, or number considered in comparative relation to a whole Items in the numerator are always included in the denominator: A Proportion: A + B 6Methods of quantifying disease

Proportion – example Live births in England, 2012: Males = 356,202 Females = 337,920 Proportion of live births that are male = 356,202. = (= 51.3%) 356, ,920 7Methods of quantifying disease

Proportion - example Blood types, United Kingdom v. Finland Blood Type English Finns O 47% 34% A 42% 41% B 8% 18% AB 3% 7% 8Methods of quantifying disease

Ratio Definition: One number compared with one or more others Items in the numerator are not included in the denominator: A Ratio: B 9Methods of quantifying disease

Ratio – example Live births in England, 2012: Males = 356,202 Females = 337,920 Male birth ratio = 356,202. = (:1) 337,920 = (:100) 10Methods of quantifying disease

Rate Definition: an event or occurrence evaluated over a period of time Number of events in a given period Rate: Total number at risk of event in same period Time element is important – the longer the time period, generally the higher the rate 11Methods of quantifying disease

Crude rates 12Methods of quantifying disease Number of cases in a population divided by total population during time interval. Provides information on the experience of the population. Useful for allocation of health resources & public health planning. However if comparing mortality rates between two populations where one population had a larger proportion of old people then differences in rates might simply reflect the relationship between mortality and age.

Crude rates - example South West, 2012 Males: Deaths – 25,331; Population – 2,619,914 Females:Deaths – 27,734; Population – 2,719,723 Crude Death Rates (per 1,000 males/females per year); Males = 1,000 x 25,331 = ,619,914 Females = 1,000 x 27,734 = ,719,723 13Methods of quantifying disease

Crude rates 14Methods of quantifying disease Cannot compare two crude rates unless know population structures the same Differences in crude rates may be due to age (and/or other) differences in population structures Age standardised rates for South West in 2012; Males = 5.77 deaths per 1,000 per year Females = 4.13 deaths per 1,000 per year

Population structures 15Methods of quantifying disease

Category-specific rates 16Methods of quantifying disease To account for different population distributions of a factor of interest we can present and compare category- specific rates These are calculated on a subgroup of the population which is defined by stratifying the populations into categories e.g. age They permit comparisons between different categories within the same population

Age-specific mortality rates 17Methods of quantifying disease South West, 2012

18 Count, rate, ratio or proportion? Methods of quantifying disease

Prevalence Definition: The number of cases of a disease or characteristic in a given population, at a specified time Number of cases at a given time Prevalence: Number in population at that time Expressed as a % or per 1,000 population etc. 19Methods of quantifying disease

Prevalence example 20Methods of quantifying disease Instant A Population 1,000 End of EpisodeStart of Episode

Incidence Definition: The number of new cases of a disease or characteristic in a given population, over a specified period of time Number of new cases in a specified period Incidence rate: Number in at risk population over same period Expressed as per 1,000 population per year etc. 21Methods of quantifying disease

Incidence example 22Methods of quantifying disease Period B Population 1,000 End of EpisodeStart of Episode One Year

Incidence - example 23Methods of quantifying disease