I am surprised I have not noticed this before, particularly since this is one of the most important statistics, and so have just added the new graphs to Watchtower Publisher Statistics.
The number of Bible Studies generally trends roughly with the number of publishers on a one to one basis. However, the number of baptisms has not been rising. This has resulted in a rapidly falling conversion rate. The percentage of Bible studies that get baptised have dropped dramatically, from a high of around 1 in 5 (22%) to now a mere 1 in 32 (3.1%).
For any JW that claims 1975 was never an important year, I'd love to see them explain that graph.
I cannot believe the following graph, when I first saw it I thought there must be a mistake in the figures. 8 million bible studies but only 200,000 baptisms looks even worse than it sounds.