Great explanations and intuitions behind basic statistics concepts
Great explanations:
- on [covariance](http://stats.stackexchange.com/a/18200);
- on [correlation](http://stats.stackexchange.com/a/18089);
- on time series [stationarity](http://stats.stackexchange.com/a/19747);
- on rule of [unit root](http://stats.stackexchange.com/questions/29121/intuitive-explanation-of-unit-root?lq=1);