Call it the start of the government’s biggest big data push.

Monday marks the start of mailings from Statistics Canada of census surveys, including the return of the mandatory, long-form questionnaire that was replaced with a voluntary survey five years ago.

Statistics Canada says more than 15 million households will receive census letters over eight days, along with reminders to either fill the form out by hand or online, which half of Canadians did five years ago.

Every home will receive a short-form questionnaire. One in every four homes will receive the long-form census.

So far this year, about 1,700 Canadians have subscribed to an online reminder from Statistics Canada to fill out their form, which the agency says requires no registration or lengthy download processes. And census officials have visited more than 60 per cent of First Nations communities since the start of April to help residents fill out the questionnaire.

The census gives a statistical snapshot of the population once every five years, collecting demographic information on every man, woman and child living in the country, as well as Canadians living abroad on a military base, or part of an embassy.

For provincial coffers, the population estimates in the census determine how much per capita funding they will receive in transfers from the federal government.

For local governments and community groups, the demographic details in neighbourhoods help with decisions on where to place new schools, transit routes, seniors’ housing and emergency services.

For companies, the census data act as a much-needed complement to what’s become known as big data.

“Some people wonder, well, why do you even need a census when we have big data?” said Jan Kestle, president of Environics Analytics.

“When you combine the kind of data we now can collect with census data, you can really get a more integrated view of what consumers want both in terms of products and services and that’s also true in terms of what citizens want from government.”

It’s a massive undertaking that is estimated to cost $715 million for the seven-year period that it takes to prepare, collect, analyze and distribute results. The final cost isn’t known until two years after census day.

The previous Conservative government replaced the long-form census with the voluntary survey five years ago in a move that caught many by surprise and lit a political fuse over the depth of data Statistics Canada collected through regular population counts. The results from the 2011 count prevented comparisons to previous years, left out some small communities over quality concerns, and raised reliability questions around response rates of immigrants and aboriginals.

As one of its first acts in government, the Liberals brought back the mandatory, long-form questionnaire.

Kestle said there will remain gaps in the data collected five years ago, but the return of the long-form census this year should bridge many of them created by the one-time absence.

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“To be realistic, of course there will be breaks (in data), but I think missing one (census) is not nearly as bad as if we hadn’t had it come back,” she said.

The long-form questionnaire will go out to one of every four households, instead of the one in every three that received the voluntary survey. Failure to fill out one of the forms could lead to a fine of $500, up to three months in jail, or both.