Game theory | Strife aquatic

The real water scandal in Rio

The bay was polluted and the diving pool turned green. But the swimming events suffered most

By H.G.

LONG before the opening ceremony at the 2016 summer Olympics in Rio de Janeiro, water conditions at the games had drawn close scrutiny. Guanabara Bay, where the sailing competition was held, is clogged with sewage, industrial waste and rubbish, and alarm bells about athletes’ safety began to ring once 13 American rowers fell ill after competing there last year. Once the games were underway, it was the indoor aquatic events that began to draw unwelcome attention. The water in a pair of swimming pools turned as green as a wine bottle during the games’ second week, after 160 litres of hydrogen peroxide were dumped into them without Olympic organisers’ knowledge. And divers began complaining about an unpredictable wind that had a habit of bursting into the open-air aquatics centre without warning. “You never know when it’s going to pick up or how strong it’s going to be,” says Michael Hixon, an American who finished tenth in the individual 3-metre final. He Chao, a Chinese diver and gold medallist at last year’s World Championships, similarly told Reuters that the wind had prevented him from making it far in the Olympic 3-metre competition.

In the end, none of these supposed fiascos seemed to matter much. Save for one Belgian who had been ailing since last month, most sailors interviewed by the press said they had no problems with pollution in the bay. And the share of individual dives in Rio that scored a shoddy 40 points or less—the kind of flop likely to be produced by sudden changes in conditions—was 3.6%, nearly identical to the percentage during the 2012 London Olympics. Although the average score for individual male diving finalists did decline from the previous games, by 3% in the 3-metre and 8% in the 10-metre, it is London rather than Rio that looks like the outlier: the 2016 marks were perfectly in line with those from 2008 and 2004. Perhaps conditions in 2012 were just unusually calm. Female divers, meanwhile, saw virtually no variation in their scores from prior games.

So was all the carping about water in Rio unfounded Brazil-bashing—just like the debunked claim by a group of American athletes that they were robbed at gunpoint? Not necessarily. Although far harder to detect than floating rubbish, discoloured water or sharp wind gusts, a subtle environmental factor may have influenced the results in at least two events: an invisible current in the pool.

On August 18th Swim Swam, a swimming-news site, published research by analyst Barry Revzin showing that competitors’ times seemed to vary consistently with their lane placement. Swimmers in higher-numbered lanes tended to go faster on their laps heading towards the start end than they did in the reverse direction, whereas those in lower-numbered lanes showed the opposite pattern. Moreover, athletes who were assigned to different lanes in different phases of an event (heats, semi-finals and finals) regularly posted their best times in the higher lane numbers and their worst in the lower ones. Overall, Mr Revzin found that swimmers in the 50-metre freestyle could expect to improve their times by 0.2% for each lane they moved over, from low to high numbers. The maximum impact—from swimmers in the first and last lanes switching places—would be a 1.4% swing for each competitor, a total difference of 2.8%. That is more than enough to push a swimmer onto or off the medal podium. A similar result was found by Joel Stager, the director of Indiana University’s Counsilman Centre for the Science of Swimming, who had previously detected an identical pattern at the 2013 World Aquatics Championships in Barcelona.

The data do not prove that these discrepancies were caused by a current. The chairman of the board of Myrtha Pools—the company that built the pool in Rio and an official partner of FINA, the sport’s governing body—told the Wall Street Journal it had tested for water movement before the games and found none. FINA itself released a statement saying it had received reassurance from the firm that “no current was detected…at any stage of the competitions”. It is of course possible, though vanishingly unlikely, that this pattern simply occurred by random chance. It is also true that in any race involving more than one lap, any such imbalance should largely cancel out: a swimmer benefiting from a current in one direction would be hindered by it on the return leg.

However, the Olympics do include two single-lap contests, in which a current could play a large role: the 50-metre freestyle for men and women. And sure enough, in these races, Mr Revzin’s model suggests that the influence of lane placement may have dwarfed the performance gaps between swimmers. Moving an athlete from lane one to lane eight, or vice versa, would change men’s times by 0.3 seconds and women’s by 0.34—equivalent to the difference between a gold medal and fifth or sixth place.

As it happened, lane assignments did not appear to determine who won the races. If we apply Mr Revzin’s correction to calculate what each athlete’s time would have been in an average lane, the gold medallists, Anthony Ervin of the United States and Pernille Blume of Denmark, would still come out on top (see chart). In fact, the only change in the men’s standings would be a swap of the bottom two places. The women’s race, however, is a different story. The silver and bronze medallists, America’s Simone Manuel and Aliaksandra Herasimenia of Belarus, both enjoyed a strong “tail wind” in lanes seven and eight. Without this advantage, they would only have wound up fifth and sixth. Instead, those podium slots would have gone to the sixth-place finisher, Ranomi Kromowidjojo of the Netherlands (pictured), who finished just 0.12 seconds off the lead despite being stuck in lane three, and the fifth-place swimmer, Australia’s Cate Campbell, who was 0.08 seconds off the pace in lane five.

Unfortunately, the gravest of the Olympics’ water-related woes will also probably be the hardest to resolve. If pre-race testing, typically by dropping a buoyant object into the pool and measuring its movement, fails to indicate a current—as Myrtha Pools says was the case in Rio—the games’ organisers will have no way to detect the issue until it is too late. A technological solution involving underwater sensors might emerge eventually. But the most straightforward fix would simply be to hold the 50-metre event in a short-course pool, half the standard Olympic length. That would force 50-metre freestylers to make a turn, neutralising most of the impact of any current. It would also fundamentally change the nature of the race and reduce times.

Whether such a drastic reform is justified depends on how frequently these patterns reoccur. At the 2017 World Aquatic Championships in Budapest, savvy fans should pay nearly as much attention to the statisticians as they do to the swimmers.

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