There’s a saying I brought with me from business to City Hall and my foundation: In God we trust. Everyone else: bring data.

Data helps uncover problems, reveal their nature, and point the way to solutions. In the market, data is power. But in government, data is often an afterthought that is buried beneath political considerations and ideological attachments. Can you imagine a manager, in any other line of work, presenting “alternative facts” with a straight face? Or dismissing the vast and overwhelming body of evidence that human activity is changing the climate? In the real world, managers who present fiction as fact, or ignore basic reality, get fired.

Good data will never replace good judgment, but it is essential to informing it. I’ve seen that time and again in both government and business.

Take air pollution, for example. When I first moved to New York, the city was choked with air pollution. It was so bad that the mayor at the time used to joke: “I never trust air I can’t see.” The air was a public health threat and an economic liability – another reason for families to flee the city for the suburbs, driving down the city’s population and tax revenue.

When I was elected mayor in 2001, the city’s air quality had improved, thanks to advancing technology and anti-pollution rules like the Clean Air Act – but not nearly enough. We confronted that challenge head-on, and when we left office after 12 years, New York City’s air was the cleanest it had been in more than fifty years.

We took a number of steps to make that improvement possible, but the most important tool wasn’t new technology. It was new data.

Through our long-term sustainability effort – called PlaNYC – we installed street-level air quality monitors to more accurately pinpoint the sources of pollution. Those sensors uncovered some surprises – and the biggest surprise of all was that just 1 percent of the city’s buildings were spewing more soot into the air than all of the city’s cars and trucks put together.

Armed with that data, our Administration banned the dirtiest burning heating fuels and created public-private funding programs to help building owners make the switch. That was the single biggest reason for the increase in air quality during our administration. In just five years, the amount of soot pollution fell by 23 percent and the amount of sulfur dioxide in the air fell by 69 percent – and that helped to save many lives and prevent many people from getting sick.

Without data, politicians don’t know what works and wind up throwing good money after bad to keep broken programs and policies in place. It’s a waste of taxpayer money and takes resources away from other important priorities that need addressing. It also takes power away from the people, because it prevents the public from holding leaders accountable for delivering results.

In other places, lack of data leads to preventable deaths and shortened lives. For instance: In the U.S., we take it for granted that everyone’s birth and death will be documented. One certificate is generated when you are born and another is when you pass away. Those documents provide essential information that governments need to design policies. But around the world, around 40 percent of all births and more than half of all deaths go unrecorded. Without that data, officials don’t know what people are dying from, which prevents them from doing anything effective to prevent death and disease, and prevents them from knowing what efforts are helping.

Bloomberg Philanthropies is working to fix that, through a global program called Data for Health. We are also helping mayors around the

U.S. use data to strengthen public services and improve people’s lives. As part of that effort, we’re helping more cities take a critical step that we took in New York City: giving the public access to city data, so that they can see what is working, what is not, and be empowered to demand better when leaders fall short.

Data is more important than ever in the era of fake news and alternative facts. We need to put more of it in the public’s hands – and use it to hold our leaders accountable.