Political candidates everywhere know there are many ways to get the word out: flooding TV and radio airwaves with ads, sending out mailers, perhaps going door to door asking for votes. There's only so much time to campaign though. And only so much money to pay for it all.

So, candidates need to know: What's the most cost-effective way to win votes? Voter mobilization has become a deeply data-driven science, and candidates are increasingly looking beyond their own borders for the best results.

When Francois Hollande was running for president of France in 2012, his campaign wanted to try something completely different. For decades, French politicians had relied on mass communication to reach voters. That’s how most modern campaigns work in the United States also.

But not always.

The Hollande campaign took note of Barack Obama’s on-the-ground mobilization efforts in 2008. Vincent Pons was one of Hollande’s campaign directors and helped mobilize 80,000 volunteers, the largest door-to-door campaign in Europe to date.

“We knocked on 5 million doors,” said Pons.

That’s a lot of three- to five-minute conversations with strangers. The doors weren’t chosen at random, though.

“They were chosen to target a specific type of voter that we wanted to reach out to, which were left-wing, non-voters, or undecided voters that we knew were hesitating between voting for the left or for the right,” said Pons.

Hollande’s staff pored over data at the precinct level to analyze voting patterns. Pons, who is now an assistant professor at the Harvard Business School, said they decided to pound the pavement for Hollande because they concluded it was more efficient than just sending out cheap mass mailings to voters.

“With door-to-door canvassing, on average, you win one vote for each 14 doors. And, I think it’s easy to understand, door-to-door canvassing involves a personal and direct interaction with someone. A mailer, often times, people will not even look at it.”

But here’s the question: Is going door to door more cost-effective? It’s expensive to train political activists. And if you can’t get volunteers, you have to pay people.

“The metric is, how many votes you win for $1?”

Or, think of it this way: How many dollars does it cost to get a vote? Researchers at Columbia and Yale, Donald P. Green and Alan Gerber, found that it costs $31 to produce a vote going door to door. That’s more than three times cheaper than sending out direct mailers (ranges from $91 to $137). Leafletting costs $47 per vote, commercial phone banking costs between $58 and $125, and volunteer phone banking costs between $20 and $35 per vote.

Given the cost-effectiveness of in-person campaigning, here’s the bigger question: Can knocking on enough doors actually win an election? If it’s close, yes.

“This can help you win a few percentage points, perhaps. But obviously it’s not going to increase your vote share by 10 percentage points,” said Pons.

So, if a candidate is way behind, it doesn’t matter how many doors they knock on.