Add link to Probability and Statistics Cookbook

Author: wxn0000

statinf.md

@@ -6,6 +6,7 @@ permalink: /statinf/
 
 - [Why degrees of freedom decrease for sample variance](https://github.com/Manu58/bias/blob/master/bias.pdf)
 - [Analysis of exponential distribution of births data set from the CDC](https://gist.github.com/ProgramErgoSum/5316008387746fcd84de)
+- [Probability and Statistics Cookbook](http://matthias.vallentin.net/probability-and-statistics-cookbook/)
 
 ## Comprehensive Notes