Thank you for your questions.
In the past, we used to use the mother tongue criterion. One of the reasons was that prior to 1971 the mother tongue question was the only question asked, along with the knowledge of official languages question, in the census. Following the recommendation of the Laurendeau-Dunton commission in the 1960s, Statistics Canada added a question in the census on the language spoken most often at home.
We used to use the mother tongue criterion because it was easy to make historical comparisons, at least from the 1950s onward. However, since the middle of the 1980s, because of the levels of immigration, which kept increasing, more and more immigrants tended to adopt English or French as their main language. It was considered by many that to use only the mother tongue variable—that is, English, French, or “other”, or “allophones”, which was used in the 1970s in Quebec—was somewhat restrictive. They wanted to take into account those who didn't have French as their mother tongue but who could use French on a daily basis at work or in everyday life. That's why the Treasury Board asked Statistics Canada to develop a variable called “first official language spoken” at the end of the 1980s—in 1989, to be exact. We derived two methods. One was adopted by the Treasury Board.
Overall, in some places it makes a very important difference. If you look, let's say, at Canada outside Quebec in general, there isn't much difference between using the mother tongue variable or the first official language spoken variable, because we have 4.1% who have French as their mother tongue and 4.2% who have French as first official language spoken. In Quebec, it makes a significant difference, because 8% of the population have English as their mother tongue, compared with 13% of the population who have English as first official language spoken. What this takes into account is the attraction that English can have on immigrants in Quebec, or used to have in the past.
When we look at some cities specifically—if we look at Toronto, for example—if we take into account the “first official language spoken” variable, more than 40% of the francophone population has French as the first official language spoken. If you take the mother tongue variable, the rate would be much lower, around 10% or even less than that. Just to give you another example, in Ottawa it's around 15%.
So using the “first official language spoken” variable is much more inclusive, if you like. It takes into account the adoption by the recent waves of immigrants who have come to Canada of one or the other official language.