EPI5.3: STANDARDIZATION: CRUDE AND AGE SPECIFIC RATES

EXAMPLE:

We will stay with the example of age, although one can adjust for any variable of interest.

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 Table 1 it can be seen that the total rates or crude rates are different for Population 1 and Population 2. Yet if we examine each age-specific stratum, the rates are identical for both populations in both strata. So something seems to be wrong here. It can also be seen that there is an extreme difference between the two populations in age structure. Population 1 is mainly young while Population 2 is mainly old. These groups die at different rates for different conditions. This is why we cannot simply compare the crude rates. So in order to compare them we must render the two populations identical as far as their age distributions are concerned.




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General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License
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