The Canadian government should rethink its decision to change the way census data are collected.

It is hard enough to get people excited about statistics at any time, let alone at the height of the summer holiday season. But in Canada this month, people have become passionate about the subject. A quiet political decision to scrap the compulsory long-form part of the Canadian census has inspired fuming in the national press, vitriolic protest from numerous academic bodies, the resignation of the head of Statistics Canada, and even a YouTube broadcast of a song espousing the importance of census data for public policy (http://go.nature.com/O9TWcf). Critics have accused Canada's ruling conservative party of being anti-science — and, worse, anti-information. To gut one of the world's most respected statistical organizations, they argue, will prevent future policy decisions on everything from health care to public transport from being based on either data or logic.

The incident comes amid a growing sense of unease about the right-leaning Canadian federal government's apparent disregard for science-based policy. The country continues to support the mining of asbestos and its export to the developing world, despite repeated calls to ban the toxic substance and cries of protest from the medical community. Canada has been one of the most obstructive countries at climate-change talks, and continues to be protective of its development of the tar sands — one of the world's dirtiest sources of oil. The federal government has fought against maintaining the supervised injection facility for drug addicts in Vancouver, despite staunch protest from the medical community and studies showing that such programmes are helpful. Now the government is threatening to undermine the system that collects the data needed for a multitude of other evidence-based decisions.

Every five years for almost four decades, the Canadian census has involved both a head count and a longer questionnaire asking about details such as ethnicity, education and housing. The long form was sent to 20% of the population, and replying was compulsory by law. At the end of June, citing concerns about privacy, industry minister Tony Clement announced that the long form would be made voluntary; an additional Can$30 million (US$29 million) a year will be spent to send the form to 30% of the population, in an attempt to make up the numbers.

“Vulnerable populations of the poor and downtrodden will be less likely to reply to a voluntary questionnaire, skewing the resulting data.”

This, as any statistician can testify, is not the same thing. Inevitably, vulnerable populations of the poor and downtrodden will be less likely to reply, skewing the resulting data. Although statisticians are adept at correcting for such factors in surveys, they can do so only if they have a gold-standard set of data to refer to — namely, the census data. Even if the voluntary data are sufficiently robust to allow for good public policy-making, it will still cause a problem for researchers looking for long-term trends, because comparing the compulsory data with the subsequent voluntary set will be nearly impossible.

In a country defined by a mosaic of immigrant groups and indigenous populations, precision data on their lives are crucial to good public policy. Statistics Canada has been widely regarded as a world leader in handling everything from the intricacies of question wording to protecting individual privacy. Although the government claims that it is responding to public concerns about privacy and 'big brother' government, this, ironically, does not seem to be based on evidence — the privacy commissioner's office has received only three complaints about the intrusiveness of the census long form over the past decade.

The United States investigated the option of switching to a voluntary long form in 2003, but concluded that it would be too expensive to bring the data up to par. Instead, this year it replaced its long form with a mandatory ongoing survey — a change that required nearly a decade of research to confirm that it would maintain the quality of the data. Academics, economists and public policy-makers find themselves open-jawed at Canada's snap decision — made without consulting the data's users — which will effectively allow the government to do less while spending more money. Letters of protest and editorials have been penned by the Canadian Medical Association Journal, the Canadian Federation for the Humanities and Social Sciences, and many others.

Canadian academics are not letting this pass quietly; the rest of the world should join their voice of protest (http://go.nature.com/dq6rZ3). It is too late to save the form for 2011, but this could, and should, be reinstated for 2016.

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