My understanding of the CPP--and if there are HRSDC people here, they can correct me if I'm wrong--is that the contributions are based on years of earnings, but that there is a formula to exclude years of low earnings or non-earnings; I think it's your seven lowest years of earnings over a 35-year period. That's excluded from the calculation of retirement benefits, recognizing the likelihood that women, for example, are likely to be out of the workforce.
On October 20th, 2009. See this statement in context.