We need, in order to do that, longitudinal data, which means we need to be able to track people for at least six years. We didn't have such data in Canada back in the seventies and eighties, so we cannot move those numbers back.
I think the general point of those numbers, though, as opposed to the exact trend, is that most Canadians don't experience low income. A substantial number do over a long period of time--20%--but relatively few experience persistent low income. That's much lower, around 4% or 5%.
But no, we don't have a time series.