Counting helmet's users

I have been doing statistics each year since 1989 counting how many bikers use helmets when going to work. I have every summer taken a sample of 100 bikers and counted how many of them have used helmet. 1989 it was 2 %, three years ago it was 73 %, two years ago  80 % , last year  85 % and today in my sample  76 % used helmet.