When Statistics Canada languages expert Jean-Pierre Corbeil sat down to look over the new language data from the 2011 census, he did a double take.

The numbers did not make sense. This is bizarre, he thought. Patterns of linguistic change established over decades appeared to have suddenly shifted.

What he was seeing is the first knock-on effect of Prime Minister Stephen Harper's decision to replace the mandatory long-form census with a voluntary survey. The impact is still hard to judge, but what's clear is that the new numbers are less reliable as a barometer of change.

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For example, the new data suggest the number of Canadians who speak neither English nor French at home grew by just 100,000 between 2006 and 2011. During that period, Canada welcomed more than 1.2 million immigrants, the vast majority of whom spoke languages other than English or French. At the same time, the number of "new bilinguals," people who said they speak English or French as well as another language at home, grew by more than a million, twice as much as in the previous census period. It is also surprising that the proportion of Canadians whose mother tongue is English held steady, since it had been in consistent decline, Mr. Corbeil said.

"When I looked at the data at first I said, "Whoops, there is something bizarre with this data," Mr. Corbeil said. "I have to admit that, yes, the trend is not the same and that's why we warn users to be cautious when looking at these trends."

Doug Norris, chief demographer at Environics Analytics, had a similar reaction. "My first thought was, 'What the heck is going on?'" Mr. Norris said.

"You've got all these people saying they speak multiple languages, multiple mother tongues plus English, so it really makes it very confusing. You can't do any sensible trend analysis as far as I can see."

The unusual results may stem from the controversial killing of the long-form census, which traditionally contained the language questions. In 2011, the language questions were put in the short form, which is still mandatory, after a failed legal challenge of the changes by the Federation of Francophone and Acadian Communities of Canada. The federation had argued that data crucial to the enforcement of the Official Language Act would be less reliable in a voluntary survey.

The wording of the questions was identical to that of years past, but residents answered them in a different context, which appears to have had a major impact on the results. In the long form, the language questions were preceded by questions about place of birth, citizenship and immigrant status. Not this time. The long form had asked Canadians which non-official languages they speak, but that question was not in the 2011 short form. It may be that the jump in the number of households that reported speaking English or French and another language – the new bilinguals – was a response to that change in context, said Réjean Lachappelle, former director of the demography division at StatsCan.

Statscan will be looking more closely at these results and producing a methodological report to explain the unusual outcome in the coming months.

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"We don't know why, we don't know what's happening, but chances are it's due also to changes in the questionnaire," Mr. Corbeil said. "Context is very important in the census. If we had questions on immigrant status and place of birth, that could have held peoples' mind focused on the information we wanted to collect. …We suspect that this might have had an influence on people's response pattern."

Canada gathers detailed language information in part to ensure an appropriate level of federal services in each region for French or English minorities. Commissioner of Official Languages Graham Fraser said he is satisfied that, with a 98 percent response rate to the short form, the numbers that are crucial to his office are accurate. What the results mean for other linguistic groups is less clear. They may overstate the extent to which some immigrants use English or French at home. Or they may reflect a new, more linguistically complex country where families move fluidly from one language to another.

Either way, the method of gathering the new data makes it difficult to assess where Canada is going in comparison to where it has been. Experts say these questions will only grow more complicated as results from the voluntary survey start to roll in next year.