Maria Salvetti. Carl Bialik A day during the tournament can involve hours of watching tennis and thousands of decisions. Salvetti arrives by 10:30 a.m. and stays until the end of play, which can be almost 10 p.m. She works one hour and then takes the next one off. On Tuesday, she worked the first, third and fourth sets of the five-set quarterfinal that was won by Jo-Wilfried Tsonga over Kei Nishikori, and she expects to work both women’s semifinals Thursday. The tournament pays her about 100 euros (about $110), after taxes, for each day of work. When she’s off duty, the last thing she wants to do is watch tennis.When they are on the job, Salvetti and her French Open crew appear to be very good. For one thing, their numbers are consistent with those of independent match loggers. I compared the official stats for eight matches from last year’s French Open and 11 from this year’s with the numbers from the crowdsourced Match Charting Project and found that the amateurs and pros like Salvetti agree. The official scorers counted just 3 percent more winners and 3 percent fewer unforced errors. Scorers at the U.S. Open and Wimbledon are far more generous, awarding on average 27 percent and 32 percent fewer unforced errors than the independent scorers, according to the dozens of matches I’ve checked.Salvetti said the toughest judgment call is whether a missed shot was forced or unforced. She must decide whether the opponent’s shot was good enough to force the error or whether blame lies mostly with the player who missed. She thinks her courtside seat gives her an advantage over scorers working from home. “On TV, you don’t see all the power that all the players put into the hit,” she said. “You don’t see all the energy they use to run from one side of the court to the other.”Salvetti cited another reason that she’s confident in her work. After matches, the scorers reconcile their numbers with those that come from the umpire’s chair. Umps don’t record winners and unforced errors, but they do take note of whether serves go in, whether they’re aces and who wins the point. Salvetti said that in the rare cases when the umpires’ numbers disagree with Salvetti and her crew’s, 95 percent of the time the scorers are right. She doesn’t blame the umpires for this: They have “a lot more to focus on,” she said.Not every tournament keeps stats on winners, unforced errors and net points, and few do for every match. When they are recorded, they don’t make it into the official stats kept by the men’s tour and the women’s tour. I asked Salvetti how she feels about that. She said she knows from her regular job as an environmental economist how important data is. She wishes more came of her hard work collecting tennis data.“All this information is not used the way it could be used, for players to know more about their games, for coaches, for even journalists and people who bet,” Salvetti said, emphasizing that she was speaking for herself and not the scorers as a group. She teaches at the Sorbonne in Paris and thinks some of her statistically gifted students could do great things with the tennis stats. “All this information, in my opinion, has value that is not used,” she said. “It’s gold that we have in our hands, and we don’t make anything out of it.” Beyond hitting the ball, tennis has outsourced a lot of its work to technology. A sensor determines if a serve clipped the net and should be replayed, radar sensors measure serve speed, and calibrated courtside cameras judge whether a shot was in and generate advanced stats. But when it comes to recording unofficial stats such as winners and unforced errors, the Grand Slams still rely on people like Maria Salvetti.Salvetti has been keeping scores and stats at the French Open each spring for the past 20 years, since she was 19. When she was a child, Salvetti trained in Paris on the courts at Roland Garros, the home of the French Open. But when she was 15, she hurt her knee, stopped playing, and found her way to scorekeeping after a stint as a French Open ballgirl. She is one of about 40 scorers at this year’s French Open. Roughly one-third of them are women, up from about one-quarter when she started, she said.The job is filled with small, fast decisions: When a point ends, scorers determine whether it was decided by a winner, forced error or unforced error that was hit as a forehand, backhand or other type of shot. They also note whether the point was won while one player was at net. The data then feeds IBM databases and powers television graphics, and journalists use it to identify how the match was won.