Was Rory McIlroy's eight-shot victory Sunday at the PGA Championship more impressive than his eight-shot win at the last year's U.S. Open?
One statistic suggests it is.
The Z-score is a statistical measure which compares a population sample against the larger population. Grantland's Bill Barnwell computed McIlroy's Z-Score for his winning performances at Kiawah Island and Congressional to to explain why he was better at the PGA:
Last year, [McIlroy's] 268 at the U.S. Open produced a Z-score of -3.07, meaning that his performance was 3.07 standard deviations below the average score of a player who finished all four rounds. He won the tournament by eight strokes, but he beat the average finisher by more than 18 strokes.
That brings us to this weekend, when McIlroy overwhelmed the competition at Kiawah Island and claimed his first PGA Championship. McIlroy's score of 275, weighed against the rest of the field, produced a Z-score of -3.15. He finished just under 15 strokes ahead of the average golfer who finished four rounds, and while he won by eight strokes, there were two dozen players within four shots of second. By a fraction of a stroke, it narrowly topped McIlroy's U.S. Open performance from last year to become the best performance of McIlroy's career.
Thankfully, stat gurus like Barnwell exist to provide some insight on exactly how impressive McIlroy's second major victory was.