Standardization - Page 4 of 7

OBJECTIVE

At the end of this session you have an understanding of direct as opposed to indirect standardisation, when it is indicated and how to do it.

DIRECT STANDARDISATION:

The reference population has been manufactured and can be any set of numbers. This will not affect the process of standardisation.

We therefore recalculate the rates for the two index populations in such as way that we ask what these rates would be if each of the two index populations had exactly the same age structure. We use the age structure of the reference population for this purpose.

**Table 1**
Age Group	Reference Population	Index Population 1	Index Population 2
Young
Cases	50	50	5
Person years	100 000	10 000	1 000
Rate	0.0005	0.005	0.005
Old
Cases	400	4	40
Person years	200 000	1 000	10 000
Rate	0.002	0.004	0.004
Crude rates
Cases	450	54	45
Person years	300 000	11 000	11 000
Rate	0.0015	0.005	0.004
Age adjusted or standardized rates	0.0015	0.004	0.004

In this scenario within each age stratum we take the rates from Population 1 (in red) and Population 2 (in red) and multiply these with the person-years from the standard or reference population (in red) respectively. This is: 0.005 x 100 000 + 0.004 x 200 000 = 500 + 800 = 13000 for both Populations 1 and 2.

13000 cases would have been experienced by Index Populations 1 (500) and 2 (800) IF they had had the same age distribution as the reference population. These are added for the two strata (to yield 1300 for each population) and divided by the total number of person-years in the reference population (300 000), which represents the total number of years of observation if Pops 1 & 2 had the same structure as the reference population. This yields the standardised or age-adjusted rate. Because populations 1 and 2 as adjusted now have the same age structure, the adjusted rates can be directly compared with each other.

It can clearly be seen that the standardised rates are identical for Pops 1 and 2 (0.004 each). This is to be expected as we have seen that the age-specific rates in the age strata are identical.

So here direct standardisation has allowed us to compare Pop1 and Pop 2 to each other by virture of the rates in both populations having been recalculated as if each of the two populations had the identical population age structure of the reference population.