I don't quite understand the things in bold:
Of the 40 occasions since 1928 in which an industry rose by at least 100% over any two-year period, a collapse of more than 40% over the subsequent two years occurred 53% of the time — or 21 times, as noted above.
There are lots of things that are confusing. Does "40% over the next two years" mean the same as "40% of the next two years"?
Does 53% of the time mean "50% of the two years"?
If so, how does the meaning of the whole sentence fit together?