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Why Mohammed Shami trumps Purple Cap holder Kagiso Rabada in Smart Wickets tally

A Smart Stats deep dive into the top bowlers so far in IPL 2020

Kagiso Rabada may be the Purple Cap holder by some distance in IPL 2020, but Smart Stats tells us Mohammed Shami is the leader in terms of wickets, not Rabada. Shami has 18.7 Smart Wickets to Rabada's 17.9. That is because Smart Wickets takes into account not just the sheer number of wickets, but also the quality of batsmen, their score at the point of dismissal, match context and pressure on the teams at that point.
Of the 16 wickets Shami has taken so far, 12 have been of the top five batsmen. Apart from these, he has dismissed the No. 6 batsman thrice, and the No. 7 once. In Rabada's case, 10 of his 21 dismissals have been of batsmen batting below No. 5, including four dismissals of Nos. 8-11. While it is not necessary that a batsman batting down the order has a lower rating - Andre Russell is a prime example - but in general, top-order batsmen do have a higher rating.
Also, nine of Shami's dismissals happened before the batsmen reached double figures, compared to just five such dismissals for Rabada. Also, wickets which came when the bowling team had already sealed the match don't count for as many Smart Wickets as when wickets are taken when the team is under pressure. For instance, in Rabada's four-wicket haul against Royal Challengers Bangalore, three of his scalps - of Washington Sundar, Shivam Dube and Isuru Udana - came after the game was all but sealed. His wicket of Virat Kohli was valuable, but not the other three. Similarly, Jofra Archer's 13 wickets is worth marginally more than Jasprit Bumrah's 15.
In terms of economy rates, Washington Sundar is the leader with a Smart Economy of 3.51, which is marginally ahead of his team-mate Chris Morris' 3.67. Smart Economy takes into the match economy rate, the phase of the game when the bowler bowled, and the pressure on the bowling team when he bowled his overs.
Sundar has been outstanding in both the Powerplay and the middle overs: in the Powerplays, while the overall economy rate for all bowlers in this tournament is 7.37, Sundar has only gone at 5.23 in 13 overs. In the middle overs (sixth to 16th), Sundar's economy rate is 5.42 the overall economy rate in these overs is 7.82. Sundar, Morris and Rashid Khan are the only ones with Smart Economy rates of less than four per over.
The overall leader in terms of bowling impact per match, though, is Bumrah (with a five-match cut-off). Bowling impact takes into account both Smart Wickets and Smart Economy, and rewards bowlers for performing in high-pressure situations. Bumrah started the season badly, but has hit top form recently: in his last six games, only once has he conceded more than 26 in four overs, and has also taken 12 wickets. In his period he has averaged 12.58 at an economy rate of 6.29.
However, it is one of the earlier games - against Kolkata Knight Riders - that best illustrates the ability of Smart Stats to capture match context in a performance. In his first three overs in that game, Bumrah went for only five runs and took the wickets of Andre Russell and Eoin Morgan. Then, with the match in the bag, he went for 27 in his fourth over, ending with figures of 2 for 32. Those numbers don't look too flattering, but Smart Stats recognized the fact that Bumrah's performance was top-notch when the pressure was high, and rewarded him with the top impact points for that match despite other bowlers having better conventional figures.
The other bowlers in the top five have also had outstanding tournaments: Yuzvendra Chahal and Archer have both been among the wickets while also being extremely economical, while Shami loses out slightly because of a higher economy rate. Rashid, while having a lower wickets tally, is second in terms of economy and third in terms of Smart Economy. Rabada is at No. 8 in this list, behind Trent Boult (sixth) and Axar Patel (seventh), while Rahul and Deepak Chahar round off the top ten.