The census question was eleven words long and it cut Canada's Jewish population in half.

According to the long-form census, the most reliable population measurement tool in the country, Canada's Jewish population declined from about 309,000 in 2011 to a little more than 143,000 in 2016. Simply not possible.

"I turned my back and half my community disappeared," said Shimon Koffler Fogel, chief executive at the Centre for Israel and Jewish Affairs.

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Mr. Fogel is understandably exasperated.

"Obviously half the Jewish community in Canada didn't disappear over a five-year period," he said. "They didn't immigrate, they didn't die, they didn't do anything. There was a flaw in the way the data was collected and processed."

The source of the problem is question 17 on the long-form census. It appeared much the same as it did on the voluntary National Household Survey in 2011. The wording was: "What were the ethnic or cultural origins of this person's ancestors?" A note, just below the question, clarified that an "ancestor is usually more distant than a grandparent," and went on to provide 28 examples of ethnic ancestry, with a space for each respondent to write in their answer. The difference was that in 2011, for the first time, Jewish did not rank among the 20 most common answers. Therefore, on the 2016 questionnaire, Jewish was not one of the listed examples. It was dropped, along with Salvadorean, in favour of Iranian and Mexican, StatsCan said in an explanatory note.

Irving Abella, an eminent historian of Jewish Canada, remembers a moment of puzzlement when he reached that question on his census form. He knew immediately that the change to the form would have a big impact, he said, but he didn't anticipate the "disaster," as he put it, that it turned out to be.

"I thought it was an egregious omission," Prof. Abella said. "It's wrong. Jews have been an ethnic group in this country for 300 years and always described as an ethnic group. Now they seem to be excluded."

Statistics Canada defended their work in a written response to The Globe's questions. They described the result as "correct." There is no suggestion, for example, that the responses were improperly recorded or tabulated, as was the case with a census finding on the anglophone population in Quebec that described a boom and was corrected to show a decline.

"The responses to the question on ethnicity are correct and reflect what respondents provided on the 2016 census," Statistics Canada said in a statement.

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"Following a long established methodology and to maintain the relevance of the examples, the list of examples provided is based on the most frequent single answers in the preceding census. The examples do not limit in any way the choice respondents make in writing in their ethnic ancestry."

The agency added that there has been an observed decrease in the reporting of Jewish as an ethnic origin over the years.

The decline was relatively slight until 2016, though, when it fell off a cliff.

There does not seem to be any shift in the way the community identifies itself that would explain even part of the result. Jewishness in Canada has always been primarily an ethnic identity, rather than a linguistic or religious identity, according to Prof. Abella. Since a question on religion is asked only every second census, and wasn't asked in 2016, it's hard to compare the result on religion with the one on ethnic identity. It's possible that some may have interpreted the question as seeking only the national origin of their ancestors, but it seems unlikely.

"We've identified as an ethnic group different from our places of origin. Jews who come from Poland will describe themselves not as Polish but as Jewish," Prof. Abella said.

"Suddenly to change that sows confusion and misunderstanding."

Jackie Luffman, writing on the CIJA website, argued that the question, as it is worded, cannot be considered an accurate way to count the Jewish population in Canada. It refers to a person's ancestors, thereby excluding all those who converted to Judaism. And those whose great-grandparents were Jewish don't necessarily consider themselves Jewish today, she added.

"As the Jewish community in general becomes more and more 'Canadian,' in the sense that we are now more likely to be third– or fourth-generation Canadian, then we may conceive our ancestors as simply Canadian over time," Ms. Luffman wrote.

She said the curious result highlights the need for a new survey of Jews in Canada that could tackle some of the complex issues around Jewish identity and culture.

The census result is not merely academic. It will have a real impact on how the Jewish community is able to understand its own population and how it allocates its resources, Mr. Fogel said.

"It has real value. I haven't quantified but certainly you're talking tens of millions of dollars annually that are allocated on the basis of the demographic landscape of the Jewish community," he added.

Mr. Fogel said that he had met recently with the minister responsible for Statscan, Navdeep Bains. He said the minister was optimistic that changes can be made in time for the 2021 census.

A spokesman for Mr. Bains confirmed the meeting and said they will be listening to the concerns that were raised.

"The minister personally met with representatives of the Jewish community," a spokesman for Mr. Bains said. "Moving forward, our government and Statistics Canada will be consulting all Canadians, including the Jewish community, as Statistics Canada goes through the public consultation process on the context of the 2021 census questionnaire."