Ananth reported that in Indian Premier League 2009, it did not matter who the bowlers were. Part-timers performed just the same as the regular bowlers on average.
Being an bowler myself, I could not imagine how could a part-time bowler perform be considered same as a professional bowler. Does it mean that there is nothing in toiling for years to perfect deliveries. At the moment all factors indicate that T20 will become the format of choice for furture. But if the quality of a bowler is not relevant in this form of cricket, in all possibility the art of quality bowling would dies and there wont be any Ashes for the bowlers.
It is clear that there is a big difference between Glenn McGrath and say Ashish Nehra when we compare them in Test or limited over cricket matches. In this post I will try to identify the differences between specialist bowlers and part-timers.
Data
I took the bowling data from cricket.org. I extracted runs conceded and wickets taken by a bowler. Subsequently I divided all the bowlers used in the T20 world-cup in four groups according to the number of overs each bowler bowled. The four groups consists of bowlers who bowled (1) 1-5 overs (red), (2) 6-12 overs (orange) (3) 13-21 overs (pale blue) (4) 22-28 overs (blue). It is possible that some regular bowlers ended up in the 6-12 over group because their team made an early exist. I did not correct for this. However, in principle we can compare the performance of bowlers with 1-5 over with rest of the bowlers.
Hypothesis
Very naively I hypothesize (and you will agree with that) that a good bowler is more likely to bowl a good over and a bad over is a less likely event. On the other hand a part-time bowler is equally likely to bowl a good over and bad over. These differences may not be captured by average economy rate or strike rate of the bowlers, because the distribution of runs-per-over and wickets-per-over are likely to be skewed for specialist bowlers and, symmetric and wide for part-time bowlers.
Bowling performance distribution
In panel A below I show the mean economy rate of the bowlers in four different groups. The average values are indeed different but the standard deviation is too big and the differences are not significant. Similarly in panel B I show the probability of a wicket. Once again the group-4 is likely to take more wickets but the standard deviation is rather high and differences with respect to the first group (part-timers) are not significant.
So panels A and B show that first and second order moment (mean and standard deviation) of the wickets and run distribution are not informative in differentiating the quality of various bowlers.
A detailed look at the data in terms of the full distribution give a complete picture. In panel D I plotted the distribution of runs given in over by specialist bowlers (blue) and part-times (red). Now the differences are much more stark.
This figure also provides support for my hypothesis that a good bowler is more likely to bowl a good over than a bad one, and the distribution of the specialists is skewed with a fat-tail. As the quality of the bowlers declines (as indicated by their use in the tournament), the distribution becomes symmetric and broader such that bowers who bowed 1-5 overs are equally likely to bowl an maiden over and an over with 16 runs.
The differences between the distributions of the specialists and part-times bowlers' economy rate is very satisfying not just because it supports my hypothesis, but also because it gives a more quantitative way to differentiate the quality of bowlers.
There are sophisticated tools to differentiate distributions shown in panel D, but at the moment I have too few data. Later when I will have a bigger database I will be able to put numbers on quality of bowlers.
Cricket.org only recently has started to put the commentary of full matches, else it would have been indeed possible to provide a quantitative difference between Ashish Nehra and Glenn McGrath beyond the averages.
right arm over
Arvind
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