I prefer using data to help understand history. Noticeable changes in relevant data indicate that something important happened here – and you should figure out what that was. I am looking at the recent history of Mormon missionary work, using the data for the number of members, missionaries, and convert baptisms published by the Church of Jesus Christ of Latter-day Saints. Some changes are easily visible, like shorting the length of a mission in the early 1980s and lengthening it again or the age change in 2012 are immediately obvious. Others, like Raising the Bar and Preach My Gospel, can be seen but are more subtle. I have no explanation for the biggest change: in the 1990s, the number of missionaries and number of baptisms decoupled. Missionaries continued to increase, but baptisms began to fall. We have never been able to get to this number of baptisms since. If we could figure out how to return to the pre-1990 relationship between missionaries and baptisms, we would have hundreds of thousands of additional converts every year.