Estimating mortality rates with direct age-standardization

Question

I'm attempting to calculate the mortality rates of AMI (acute myocardial infarction) in patients (cases) with high bloodpressure, between the years 2001 to 2008. For every case I have 2 matched controls that are healthy (i.e normal blood pressure; matched for age, sex, socioeconomic status and demographics. Both controls and cases have been divided into 4 different age-groups. Thus far, I have calculated the number of events and person-years for both cases and controls.

I use the epitools package in R and the ageadjust.direct function.

I'll attach the example from the description in the epitools package:

population <- c(230061, 329449, 114920, 39487, 14208, 3052,
72202, 326701, 208667, 83228, 28466, 5375, 15050, 175702,
207081, 117300, 45026, 8660, 2293, 68800, 132424, 98301, 
46075, 9834, 327, 30666, 123419, 149919, 104088, 34392, 
319933, 931318, 786511, 488235, 237863, 61313)
population <- matrix(population, 6, 6, 
dimnames = list(c("Under 20", "20-24", "25-29", "30-34", "35-39",
"40 and over"), c("1", "2", "3", "4", "5+", "Total")))
population


count <- c(107, 141, 60, 40, 39, 25, 25, 150, 110, 84, 82, 39,
    3, 71, 114, 103, 108, 75, 1, 26, 64, 89, 137, 96, 0, 8, 63, 112,
    262, 295, 136, 396, 411, 428, 628, 530)


count <- matrix(count, 6, 6, 
    dimnames = list(c("Under 20", "20-24", "25-29", "30-34", "35-39",
    "40 and over"), c("1", "2", "3", "4", "5+", "Total")))
    count

### Use average population as standard
standard<-apply(population[,-6], 1, mean)
standard


birth.order1<-ageadjust.direct(count[,1],population[,1],stdpop=standard)
round(10^5*birth.order1,1)

I've done some age-standardised mortality rates in SAS with Proc STDRATE. I don't know exactly how the Proc STDRATE function works but my guess is that it somehow computes the mean age in all age-groups during the entire study period and subsequently works with these to estimate the IRR.

My questions;

• In the example in the age-standardized description they've computed the mean persons-years for every age-group and use that as the stdpop=standard instead of using the mean age of the respective age-group. Intuitively I would say that the mean age of every age-group would be more correct (since were trying to standardise for age) then the persons-years.

• I's it theoretically correct to use my controls as the reference population as they are already matched for both age and sex or should I instead use the mean populations age as my reference population.

I might have misunderstood the argument stdpop in the ageadjust.direct function, if so I would be glad if someone could shed some light on that too.