R Ashwin and Ravindra Jadeja are more successful as a pair than Anil Kumble and Harbhajan Singh were - it's a fact. After all, India have now won 25 of the 33 Tests Ashwin and Jadeja have played together as compared to just 21 out of 54 for Kumble and Harbhajan.
So, are they are just better match-winners? Or are there other explanations for the fairly large statistical gap between the two pairs? Harbhajan has himself tweeted about how pitches in India offer more spin now than before. It's also true that Kumble and he played more Tests together overseas. Then there is a difference in the quality of India's support bowling cast. And, perhaps, opposition batsmen were better against spin then than they are now.
So how do the numbers compare when we try to take at least some of these factors into account?
The home-and-away factor
First, we limit the comparison to home Tests. Kumble and Harbhajan played 20 Tests outside India, while Ashwin and Jadeja have played just four. With India finding a set of impressive pace bowlers over the past decade, there has not been a need to play two spinners in places such as South Africa, England and Australia. But, even in home Tests, Ashwin and Jadeja have a superior bowling average and strike rate than Kumble and Harbhajan. The team has also been more successful of late: India have won 22 and lost only one out of 29 Tests Ashwin and Jadeja have played together at home. In comparison, India won 14 out of 34 with Kumble and Harbhajan in the XI.
Who got the friendlier pitches?
Have Indian pitches been more spin-friendly in the Ashwin-Jadeja era as Harbhajan feels?
It's hard to measure, but when you look at the records of other spinners - both for and against India - playing in the same games as Kumble and Harbhajan, they averaged 41.11 compared to the 42.82 that spinners playing alongside or against Ashwin-Jadeja have.
Before concluding that the pitches have been more or less the same, let's look at who we are talking about. When Kumble and Harbhajan were playing, Shane Warne, Muttiah Muralitharan, Saqlain Mushtaq and Danish Kaneria all toured India. Of these, only Saqlain outbowled the Indians. During Ashwin and Jadeja's time, the quality of visiting spinners has not been nearly as good, but when Nathan Lyon came with Australia in 2017, Ashwin and Jadeja, as a duo, did better than he did. It's hard to say what Muralitharan and Warne may have done on the pitches India have prepared over the past few years - not to mention the difference in the quality of the Indian batsmen too. Equally, it's tough to measure whether batsmen of the past would have played Ashwin and Jadeja better.
The support cast
One significant difference between the time Kumble and Harbhajan bowled together - 1998 to 2008 - and now is that India's seam attack is much better. Ashwin and Jadeja have Ishant Sharma, Mohammed Shami and Umesh Yadav taking key wickets even on dry tracks to take the burden off. Thanks, in part, to their own batting skills, Ashwin and Jadeja have also had the advantage of, at times, playing with a third spinner, and the Yadavs Jayant and Kuldeep have both been competent allies, the latter even outperforming his senior team-mates on occasions.
During Kumble and Harbhajan's time, the rest of India's bowlers averaged 41.40 at home. So it was often down to the two of them to win games. That goes a long way in explaining the much lower win percentage India had then. When you compare Ashwin and Jadeja to their bowling colleagues, and Kumble and Harbhajan to theirs, it's the older duo who actually come out looking marginally better, averaging one-and-a-half times less than their team-mates and striking at a much better rate too. Ashwin and Jadeja's support bowlers actually strike at a decent rate of 62.6 balls, which tells you a lot about India's overall bowling improvement.
Clearly, there are several factors that have worked in Ashwin and Jadeja's favour. But it might be incorrect to say that they have thrived only in tailor-made conditions, as evident from the averages of other spinners on the same tracks. But so many intangibles - they make forming a solid argument one way or the other difficult. That said, with a win percentage of 75.75, it's hard to argue against the impact Ashwin and Jadeja have had.