I've been having a lot of discussions about finance recently in which people will point to some results using overlapping time periods and claim a high degree of statistical significance. For example, they might say only one 20 year period since 1900 (1900-1920, 1901-1921, etc) has experienced X. They will say they have 91 data points. I say they have about 5.5 independent data points and that using overlapping periods overstates the statistical significance of the results.
1) Am I missing something?
2) Is there any respected source I can point to on the subject, such as a widely used textbook?