# Note - age determination

Gross-scale mean ages were calculated as follows:

1. All skiers in each age group were assigned the mid-point age in each age group (e.g., 42 for age group 40-44).

2. Mean ages for a certain category, e.g., classic racers, were calculated by multiplying the number in each age group x mid-point age of each age group, summing these products over all age groups, and then dividing this sum by the total number of racers in all age groups within the category.

3. Gross-scale means did not include those younger than 18 or older than 79. There were a few skiers <18 in 1999 and 2001, but Birkie skiers since then have had to be at least 18. There were very few in the 80+ category, so their exclusion does not have much effect on summaries.

As long as the individual ages within each age group are normally distributed around the mid-point age, which will be the case with large datasets, the gross-scale mean ages will be pretty accurate.

An empirical test of the accuracy of this procedure used the results from the years from 2002-6, which included actual ages in the results PDF files. Using these data, it was possible to compare the actual mean age to gross-scale mean age for the following classes:

Mean Age Gross-scale Mean Age

Female - classic 43.3 43.3

Male - classic 47.2 47.2

Female - freestyle 38.9 38.9

Male - freestyle 43.4 43.4