Your questions are far-reaching. I'll try to answer them as quickly as possible.
I'd like to begin by making a correction: a census does not give a picture of a situation from five years ago. The 2016 census gives us a picture of language behaviour, no matter what the province or language group is, at the time of the census. This picture allows us to study changing behavioural patterns from one census to the next. For example, we can compare a cohort of immigrants from census to census. I think it's important to point that out.
For public language, the only information the census provides is language of work, and it has only done so since 2001. It's true that this is a very short period for identifying a trend. As it happens, this one is not very positive. Quite the contrary.
It's worth emphasizing once again that language of work is a serious problem. One of the indices that deserves attention is first official language spoken, often referred to as FOLS. The first official language spoken is chosen on the basis of your knowledge, your mother tongue and the language used at home. It's the language you speak when you leave the house and go out in public. According to Statistics Canada, the ability to speak this first official language in public has declined considerably since 2001.
Moving on now to your next question about the data used to make forecasts, these projections or forecasts have proved to be accurate so far. Whether for fertility, language behaviour, international immigration or any scenarios underpinning them, they have all proved to be highly accurate.
We can therefore assume that what we forecast for 2021 will basically be confirmed by the 2021 census. That, moreover, is what has been the case for all the forecasts we've made in the past. I have been making them for approximately 40 years now. We have always correctly forecast trends, although occasionally they proved to be correct more quickly or more slowly than we thought. But the fact is that the trends have always been corroborated. I would therefore be very surprised if the 2021 census, for which we will soon have the results, would not do likewise.
I'm not sure whether I've answered all your questions.