What is the size of an average penis?

There have been several studies regarding the average size of the human penis. The majority of such studies could be unreliable due to self-selection bias: men with a smaller than average penis might be less likely to allow themselves to be measured, while men with a larger than average penis might be more likely to allow themselves to be measured. It is unknown if this conjectured self-selection bias has been confirmed by any studies, or if any studies have attempted to account for potential self-selection bias regarding penis size.