New balls, please
The Quarter finals are underway at Roland-Garros, the only clay-court Grand Slam of the ATP/WTA tour. In less than one week the French Open champions will be decided on the Phillipe Chatrier center stage. At this point on the men's side, other than a surprising first round loss of 6th seed Thomas Berdych (to homegrown Stephane Robert) and the second round loss of 8th seed Jurgen Melzer, players have been true to their rank. The course is set for a quarter final contest among the four top players in the world: The Claymore (Andy Murray) against El Toro (Rafael Nadal); Basel's own (Roger Federer) against the great hope from Belgrade (Novak Djokovic).
With so few surprises in the tournament play, what might end up being the greatest controversy of the 2011 French Open is an event that took place before any player placed a sneaker onto red earth. The event at issue was the decision of the FFT (Fédération de Tennis) to introduce a new type of tennis ball. The year 2011 marks the beginning of a five-year partnership with the Lyon-founded tennis equipment company Babolat, the newly crowned official tennis ball supplier for Roland-Garros. What would seem an innocuous innovation has stimulated many after-match grumblings to the press. Some competitors have been candidly unnerved with the switch from last year's Dunlop ball.
Federer and Murray have each expressed frustration with the change. It was not so much out of a preference for the speed and control with other designs but more out of a wish for consistency across tournaments. "Now we're stuck with a different deal for all the different ATP Tour events" Federer told reporters. Sentiments echoed by Murray "I would just rather we played with the same ball throughout each part of the season."
Readers might be surprised that an expectation of consistency would come from the sport of tennis. After all, in terms of conditions, the sport is fundamentally inconsistent. Players are asked to compete on four different surfaces, in more than 20 tournaments scattered throughout the globe (the shortest distance between two consecutive competitions at a 500-level or higher is the 618 kms separating the Barcelona Sabadell and the Mutua Madrid). The single elimination play has no sympathy for what adjustments the fluctuating conditions might require. Apart from the Davis cup and the London masters, players are given no second chances. To get to this level, tennis professionals have had to master the art of adaptation, a fact which is difficult to square with all the fuss over a new ball. But perhaps it is for the very reason that players are asked to contend with so many variable factors that throwing one more novelty in the mix has over-tried their patience.
Of greater concern is the suggestion by some competitors that the Babolat balls have actually altered the dynamics of play. Many agree with Federer that "the balls are very fast", which should favor players with big serves and powerful ground strokes. But players are also saying that the balls are harder to control, making it unclear whether bangers could capitalize on the greater speed of the new equipment, since they might not be guaranteed to hit the intended target. Djokovic summed up the uncertainties well in this reflection on the controversy: "Maybe it's going to favor the servers and the big hitters. But you never know."
Still, can we really never know? Surely, if the new balls have shifted the nature of play, it should be reflected somewhere in the numbers. Faced with quandaries like this, the statistician almost reflexively hears the tap tap tap of Karl Pearson's authoritative finger against the desktop and recognizes in it the implicit request for statistics on the table. In other words, we will let the data decide whether the Babolgate is real.
If the new balls have given an edge to servers, the service advantage will be even greater at Roland-Garros 2011 than in past tournaments with otherwise similar conditions. Stating this prediction is easy enough. The tricky part is defining similarity and in choosing a comparison group that can isolate how the one factor of the Babolat ball has influenced server performance. Considering the service advantage of the winners of the round of 16 (the last completed round at the time of this writing), our selection should control for surface, recency and stage of play. This is best done by looking at the same round of play for Roland-Garros 2010 and for the major lead-up tournaments to this year's slam.
The numbers for the 1st service winning percentage among these victors are presented as a forest plot, ordering the performance from lowest to highest (Figure for 1st Service). The forest plot was developed for use in meta-analysis as a tool to help see "the forest through the trees" when multiple studies have tested the same hypothesis. Here, each player is like a study, and each round of 16 match is an estimate of the winning player's service advantage. The lines and size of the tennis balls (green symbols) reflect the estimate's precision. When there were fewer service games, the intervals are large and the tennis balls small, to reflect greater uncertainty.
If Babolat has juiced their tennis balls, we should see all the French Open stats of 2011 cluster at the bottom of the figure, where the winning percentage is highest. We do find that Nadal, Djokovic and Federer all improved on their R16 first serve dominance this year, but the change is so small that it could be explained by chance. An estimate of the ball effect on the average 77% service advantage, after removing the effect of best of 5 matches at the grand slam, was less than a 1% increase.
Results for the 2nd service winning percentage are more compelling (Figure 2). They show a statistically significant 10% drop in the 2nd serve dominance at this year's French Open, due mainly, but not exclusively, to the lower-ranked winners. Because the same effect was not found with 1st service game, it would be hard to attribute such a huge difference to other factors that are also unique to Roland-Garros 2011, like windy conditions or Djokovic's phenomenal pursuit of the number one world rank. All indications from these "stats on the table" suggest that players are struggling to adjust to an unwieldy Babolat ball.
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