Never before have we had so much information about the world’s billionaires.

We know their average age (63), their marital status (90 percent are married, 6 percent divorced), their most common alma mater (Harvard) and even their most common sign under the Chinese zodiac (the snake). We can get daily updates on their net worth and their competitive positions within the wealth rankings.

An entire cottage industry has grown up around probing and publicizing billionaire wealth. At least six different companies now compile billionaire lists.

“Look, the billionaires control more than 4 percent of the wealth on the planet,” said David Friedman, president of Wealth-X, a private company that tracks and ranks billionaires. “There’s a lot of interest in who they are and how much they have. As long as there has been wealth, there has been a fascination with who has the most.”

Yet determining how much the billionaires have — and exactly how many there are in the world — depends on who’s counting. The fast-growing number of rich lists has produced wildly different sets of numbers on the population and wealth of the world’s billionaires. While one company says there are more than 2,400 billionaires in the world, another says there are about 1,800. One says China has the most and that Beijing is the world’s billionaire capital, while another says the United States has twice as many as China.

Tallies of individual fortunes vary even more. One ranking pegs the Ikea tycoon Ingvar Kamprad as the 10th-richest man in the world, with a $42 billion fortune. Another ranking says he’s not even a billionaire. The Koch brothers, David and Charles, are each worth either $43 billion or $55 billion, depending on the source.

The Kochs are doing just fine either way, of course. And whether there are 2,400 billionaires or 1,800 hardly matters to most of us. Yet in the age of rising wealth concentration, the financial lives of billionaires have attained outsize importance to policy analysts, inequality crusaders and wealth voyeurs alike. And measuring the wealth of the wealthy, it turns out, is often more art than science.

"Some of the numbers have to be taken with a grain of salt,” said Caroline Freund, a senior fellow at the Peterson Institute for International Economics. "But the estimates have value because it tells us what activities are most rewarding and where the billionaires are coming from. A best estimate is better than no estimate with wealth.”

Forbes magazine started the rich-list craze in 1982, when it published its first “400” list of the 400 richest Americans. It followed in 1987 with its World’s Billionaires List. In 2010, two Forbes staff members left to help start Wealth-X, a private wealth research firm also devoted to calculating the fortunes of billionaires and multimillionaires. The company now has over 170 researchers compiling detailed “billionaire dossiers,” which it sells to private banks, private-jet sellers, luxury retailers, universities, charities and other groups looking for clients or donors.

Wealth-X keeps its own finances a secret, but says its business has tripled over the last three years. The company says its larger army of researchers has helped it identify more than 2,400 individual billionaires in the world, compared with Forbes’ 1,864. (Forbes says it tracks an additional 500 billionaire “family fortunes.”)

“The quality of these lists comes down to resources,” Friedman said. “The more people you have working on it, the better your numbers are going to be.”

In 2012, Bloomberg started publishing its real-time billionaires index, which is updated daily. And the Shanghai-based Hurun Report, a wealth research firm founded in 1999 by the British accountant Rupert Hoogewerf, has also started tracking billionaires worldwide after starting out with its China rich lists. Hurun publishes more than 20 magazines and supplements a year and makes money from a fast-growing series of conferences and events.

The battle of the billionaire lists can grow contentious. In February, Hurun said that the 100 billionaires in Beijing made it the new “billionaire capital of the world” and that China had 568 billionaires to America’s 535. Forbes shot back in an article stating that New York was the billionaire capital, with 79, while Beijing ranked fourth with 51. Forbes and Wealth-X say the United States has more billionaires; Wealth-X tallies 585 in the United States to China’s 260. Forbes said Hurun sometimes included an entire family’s fortune when identifying a billionaire, while Forbes counted only individuals.

While the lists are unanimous in crowning Bill Gates the richest man in the world, their valuations for him range from $75 billion to $85 billion. The Spanish retailing giant Amancio Ortega ranks second with about $74 billion.

Billionaires themselves often disagree with their public valuations. Some seek to be removed from the lists, fearing the exposure, and argue that their net worths are far lower. Others argue that their public valuations should be far higher.

“Billionaires owe their success to being competitive, so they’re competitive about where they rank on the billionaires lists,” Friedman said.

None may be more competitive about their wealth ranking than Donald Trump. Despite Trump’s claims that his net worth exceeds $10 billion, all of the wealth research firms put him far lower in rankings. While Hurun lists Trump’s wealth at $6.5 billion, Forbes says it is $4.5 billion and Wealth-X has it at $4.5 billion. Bloomberg pegged him at $2.9 billion, which Trump called “a disgrace,” blaming Michael Bloomberg.

As for Bloomberg, Forbes put his net worth at $45 billion. But Bloomberg doesn’t appear on Bloomberg’s own billionaire list. The company says its policy “is not to cover” Bloomberg’s company, Bloomberg LP.