Spin bowlers pre-1990s tend to have much higher strike rates and much lower economy rates than spinners in today's game. At first, I thought that this was to do with the increasing pace of the bowlers, but I noticed that both Bishan Bedi (a slow spinner) and Derek Underwood (a quick spinner) have strike rates over 70 and economy rates under 2.15. Bhagwat Chandrasekhar breaks the trend with an economy of 2.70 and strike rate of 65.9, but even these are lower and higher respectively than my following two examples.
Compare this to Harbhajan Singh who has a strike rate of 65.6 (economy rate of 2.83) and Anil Kumble who has a strike rate of 64.8 (economy rate of 2.83). The idea of spinners of the past being simply, better bowlers who pushed the batsman on the defensive is mooted by Muralitharan, who is clearly at least on par with the spinners of the past - he has a strike rate of 54.4 and economy rate of 2.41.
What do you think is the cause of the increase in economy rate and decrease in strike rate of spin bowlers? Although uncovered pitches would create a lower economy rate of the more older bowlers, it would not account for those post the introduction of covered pitches nor should it increase the strike rate of the older bowlers, should it?