Standardisation Alexander Ives Public Health England, South West

Learning Objectives To understand the need for standardisation and know how to apply it Two methods: – Direct – Indirect

Crude Rates Crude incidence/mortality rates Measure of the number of new cases/deaths due to a disease in a given time span i.e. Number of cases/population Usually presented as an annual rate and quoted as number of cases per 100,000 of a defined study population.

Crude Rates 2

Age Specific Rates Age specific rates One approach to resolving the problem of different population age structures is to compare rates within specific age groups.

Age Specific Rates 2

Example of London and Camden LondonCamden AgePopDeathASDR per 1,000 PopDeathsASDR per 1, ,0005, , ,500,0005, , ,000,00010, , ,000,00015, , ,000,00025, , Total8,000,00060, ,0001,3006.5

Percentage of the population by age group

The problem… The crude rates are not comparable because the age structure of the populations are different What would the expected number of deaths be in London if the population age structure of London was standardised using a standard population. This is DIRECT STANDARDISATION called DIRECT STANDARDISED RATES If we used the same standard population for Camden, the rates for London and Camden are comparable.

Direct Standardisation Age specific rates of the study population are applied to a standard population Gives an age adjusted rate Overall rate that would be observed if the study population had the same age structure as the standard population

Direct Standardisation Method 1.Decide which standard population to use 2.Multiply each study population age specific rate by the standard population figure 3.The sum over all age groups gives the total number of expected deaths 4.Divide by the total Weights of standard population. 5.Calculate ASR for males and females separately. Person ASR = (male ASR + Female ASR)/2

European population structure

Direct standardisation Example of London and Camden LondonCamden Age ASDR per 1,000 A Euro pop B Expected deaths A*B ASDR per 1,000 A Euro pop B Expected deaths A*B Total7.51   11.3 DSR9.0/1 = /1 = 11.3

Interpretation of Directly Age Standardised Rate Weighted average of age specific rates Expected number of deaths if both London and Camden had the same population structure as the European standard

Exercise 1 Direct Standardisation This exercise refers to the excel spreadsheet Standardisation _exercise Using the Age Specific Rates worksheet, calculate the following: 1. Male lung cancer five year age-band age specific rates 2. Male lung cancer crude rate 3. Male lung cancer crude rate for 75+ years only. Using the Direct Standardisation worksheet, calculate : 1. Directly age standardised lung cancer incidence rate for males, females and persons

Indirect Standardisation Age specific rates of the standard population are applied to the study population Observed compared to expected usually as a ratio – Standardised Mortality Ratio (SMR). What would the expected number of deaths be in London if the age specific rates of England applied? This is INDIRECT STANDARDISATION called STANDARDISED MORTALITY RATIO

Indirect standardisation – method 1.Decide which standard rates to use 2.Multiply each standard age specific rate by the study population figure 3.Sum over age groups to give total number of expected deaths 4.Express as a ratio of observed to expected deaths multiplied by 100 to give the SMR

Indirect standardisation Example of London and Camden LondonCamden Age England ASDR per 1,000 A Pop B Expected deaths A*B/1000 England ASDR per 1,000 A Pop B Expected deaths A*B/ , , ,500, , ,000, , ,000, , ,000, , Total8,000,000  ,000  909 SMR60,000/65,350 * 100 = 921,300/909 * 100 = 143 SMR for England = 100

Interpretation of SMRs/SIRs SMR < 100 : lower rate than expected SMR = 100 : Expected/standard rate SMR > 100 : higher rate than expected An SMR of 180 represents a mortality rate that is 80% higher than expected. Note: Need to take into account confidence intervals

Aliases SMR (Standardised Mortality Ratio) SIR (Standardised Incidence Rate) SAR (Standardised Admission Ratio)

Pros Can use where diseases are rare Don’t need local event information for all age groups Just need total number of observed and expected counts Cannot compare SMRs with each other unless population structures are identical Cannot look at trends through time Cons Pros and cons of SMRs

Exercise 2: Indirect Standardisation This exercise refers to the excel spreadsheet Standardisation_exercise Using the Indirect Standardisation worksheet, calculate : 1.The expected number of male lung cancer cases in practice A had England age specific rates applied. 2.The Indirect Standardisation Ratio 3.How would you interpret this Ratio?

Which method to use? If want to compare several population groups or several time periods use DIRECT as with INDIRECT can only compare each population group to the standard. INDIRECT is useful to determine if disease incidence is high or low in one area only. If age specific rates for the population groups are not available or unreliable use INDIRECT. If it is a rare event and therefore number of deaths in population groups is small (e.g. ward level CHD deaths) use INDIRECT.

Some Questions…. Which type of measure would you use if 1.You are trying to monitor a trend 2.Trying to pick out which wards have a mortality rate higher than the regional average 3.Looking at infant mortality 4.Looking at the number of people with diabetes

New Version of European Standard New European Standard age structure based on 2013 population. The older age groups have a greater weight compared to the 1976 European Standard Population This change to the methods for the calculation of ASRs will have a significant effect on a variety of figures published as official statistics in the UK, by ONS and other Government Statistics Service (GSS) organisations. Age Standardised Rates using the 2013 European Standard can be approximately be double the Rates using the 1976 European Standard!

Finding out more: APHO