In statistics (in particular time series analysis, like ARCH/GARCH models) I've noticed that residual diagnostics usually look at autocorrelation of residuals and squared residuals. Why both? What ...
This story yesterday (no need to follow the link to understand the question!) http://www.cnn.com/2011/US/02/01/texas.oldest.person.dies/index.html?hpt=T2 reminds me that I've often wondered about ...