Here, the "in which" clause is giving a description of the word "financial cycle". What is the financial cycle? It is credit growth, which is followed by a bust (a sudden collapse), which is followed by new credit growth. Here are more examples of "in which". Yesterday, I saw a movie in which a man fought a dinosaur. This book, in which a man fights a dinosaur, is interesting. ( =this book is interesting. In the book, a man fights a dinosaur. ) So, now that we know what a financial cycle is, we can ignore the "in which" clause. Look at the simpler sentence: "Since the 1980s, the financial cycle has grown in amplitude but has kept its length at about 15-20 years." That means that a bust happens every 15 to 20 years, and the cycle starts over again. There might be a bust in 1980, 1995, 2010, 2025, etc. However, the "amplitude" of the cycle is increasing. That means that there is more growth at the beginning, and more severe busts at the end. So, let's say in 1980 the stock market lost 5% of its value, in 1995 it lost 10%, in 2010 it lost 15%, and in 2025 it will lose 20%. Obviously these are not the real numbers, but they should show you what the sentence means. "the 1980s" is the entire period between 1980 and 1989.