FanGraphs founder David Appelman writes the formula for FIP or fielder independent pitching which estimates a player’s ERA based on strikeouts, walks and home runs on a mirror in his Arlington, Va., home office. (Katherine Frey/The Washington Post)

A decade ago, the serious baseball fan – serious, as in desperately wanting to increase his or her chances of winning his or her fantasy league – had few places to turn. Bill James’s annual “Baseball Abstract” offered groundbreaking analysis through the late 1970s and ’80s, but the concepts outlined in those books — serious baseball analytics that might predict the performance of actual players – were scattered here and there across the Web. Baseball blogging was in its formative stages. Most fans still hung onto the ideas that wins for pitchers and RBIs for hitters were enormously important.

And there David Appelman sat, home from college, living in McLean with his dad, frustrated with his job. To get ahead in his fantasy league, he regularly read the analysis on a Web site called BaseballHQ.com. As he sifted through the data, he thought he could apply the skills he was using at work – graphing all manner of information for AOL – and apply it to baseball. In August of 2005, to the notice of just about no one outside of his family, he launched FanGraphs.com.

“It was sort of a side project, but I hoped it would turn into a business,” Appelman said. “I wanted to make it a real thing in case it could be.”

It is, indeed, a real thing. Those first few days, the site received three or four hundred hits, primarily because the blog Baseball Musings gave it a small plug. By the following summer, when it received about 1,000 visits a day, Appelman quit his job.

“It was probably a pretty nutty idea at the time,” he said. But in 2015, FanGraphs, which Appelman runs from his Arlington apartment, averaged roughly one million unique users per month, users who accounted for about 13 million page views, he said. And these are some of baseball’s most engaged readers, spending an inordinate amount of time poring over the statistics and analysis available — including regular visits from front office members of nearly every major league club. One number Appelman has been using recently: Last year, FanGraphs users spent more than 300 years of total time on the site.

“I like to think of FanGraphs as kind of the pinnacle of the online sabermetric community,” said Josh Weinstock, an analyst in the Washington Nationals’ baseball research and development department. “I really view it as the focal point through which people can more rigorously study baseball in a more quantitative way.”

The rise of Fangraphs — should it be put in a, well, graph – would mirror the greater availability and understanding of the use of advanced analytics in baseball. James, now a member of the Boston Red Sox front office, long ago coined the word “sabermetrics”, an homage to the Society of American Baseball Research (SABR), define the statistical analysis of baseball. The term is so commonplace now that it has a spot in Merriam-Webster, and there are myriad sites and companies and clubs devoted to its discussion and development.

Almost every major league team has some sort of analytics department, and the staffing of those departments has exploded not just in the last decade, but in the last two years. FanGraphs and other sites like it – including Baseball Prospectus and The Hardball Times, each of which has had writers hired to serve in the front offices of major league teams, and baseball-reference.com – have contributed not only to changing how baseball teams think about analysis, but how fans think about their favorite players and teams.

“There’s no question the average fan is better educated in baseball and understands baseball better than even five years ago, but certainly 10 or 15 years ago,” said Ben Jedlovec, the president of Baseball Info Solutions, the company through which FanGraphs and several other sites and media outlets – not to mention 22 major league teams – get their data. “There’s been such a development in baseball analytics. To David’s credit, he’s very quick to adopt other people’s ideas. He’ll talk to them, learn, and then get to work on the site and have it there for his readers. As a result, FanGraphs has been one of those places you can go to for everything.”

But such depth and breadth of information wasn’t as widely available, or as widely understood, in 2005, when Appelman, now 33, approached Baseball Info Solutions wanting to buy its data. The company was collecting all manner of information from every game – the placement of every hit, the speed and break of every pitch – and selling it to teams and media companies.

Appelman, too, became a customer, and the data from BIS formed the backbone of FanGraphs when it began. The site featured basic stats that were updated daily, a key for fantasy baseball players. It had more advanced info on batted balls – fly-ball rates, line-drive percentages, etc., tools that were starting to be used in major league front offices to better understand and predict performances in a way batting averages and RBIs couldn’t. And it contained 10 graphs providing information on every player – graphs featuring such stats as strikeout rates, walk rates and home run rates for pitchers.

“I launched, and I was like, ‘Now what?’” Appelman said. “There was no blog. There was nothing.”

Eventually, Appelman started writing essays meant to explain a particular graph, showing why a player might get better or worse in the future. This, though, grew repetitive and tiresome. So Appelman made a bold decision: He was going to hire writers to help explain the stats, and he was going to pay them. Yes, it was $10 per post. But it felt professional.

“Especially in the mid-2000s, there weren’t too many places to be a baseball writer,” said Dave Cameron, who back then ran a popular blog focused on the Seattle Mariners and accepted Appelman’s offer. “It just wasn’t a logical career path to consider.”

Cameron’s blog, called U.S.S. Mariner, delved deeply into contract analysis, disputing widely held notions of what players were actually worth. Appelman wanted Cameron to practice similar analysis using FanGraphs’ growing array of stats to back it up. In 2008, with help from several other sources, Appelman and Cameron used that kind of thinking to put a new stat, Wins Above Replacement, on the site. WAR, as it is commonly known, is designed to be an all-encompassing valuation of a player, taking into account offense, defense and base running for position players. The number WAR’s formula spits out represents the number of wins a certain player provides above what could be expected from a player that would be called up from the minors.

Baseball-reference.com has its own version of WAR, and Baseball Prospectus’s version is called WARP (with the “p” standing for player). Whatever the source, though, the evaluation gained credibility even as fans and front offices picked apart the criteria. Appelman foundusers were drawn to it. In 2009, the site grew by about 250 percent, he said.

“Adding wins above replacement, that turned out to be a very big thing,” he said.

Cameron eventually quit his job as an accountant so he could finish up his economics degree, simultaneously cranking out two posts a day. In 2009, Appelman flew to Seattle for a U.S.S. Mariner social event Cameron had organized. The two had never met. By the time Appelman flew home to Washington, he had offered Cameron a job: FanGraphs’ first full-time writer.

“I remember telling David jokingly that I was trying to buy stock in the company,” Cameron said. “I thought this could really turn into something that I feel invested in.”

Now, FanGraphs has eight full-time writers, and like other similar sites tries to find the best young analysts available. Kiley McDaniel, who wrote intensely about prospects for FanGraphs, was hired last fall as the Atlanta Braves’ assistant director of baseball operations. Weinstock wrote several articles for the site as a high school student, articles that caught the attention of Sam Mondry-Cohen, the Nationals’ director of baseball research and development. As a college student at Emory, Weinstock interned for the Nationals. By late 2014, before he graduated, he was hired full-time.

“What really resonated with me was this quest for where the real signal is in baseball,” Weinstock said. “What is the most objective way we can learn about the game that we feel so passionately about? FanGraphs seemed really interested in looking at that, in developing new ways of measuring talent. It still does.”