When asked how long a software development task will take I've started giving estimates with a probability that it will be completed within that estimate. This lets me communicate a confidence in the estimate, i.e. 80% chance of fixing a bug in 3 days. If I know little about the work to be done then the confidence might be much lower to the point where some work needs to be done just so a more useful estimate can be made.
This is useful for one task at a time, but can multiple estimates and probabilities be usefully combined?
For example: $$ \begin{array}{c|c} \text{days} & \text{percent}\\ \hline 3& 50\\ 9& 80\\ 6& 90\\ 2& 20\\ 5& 60\\ \end{array} $$ My first thought is to add the days and average the probabilities to give 25 days and 60%, but this feels like it's giving too much weight to the small task.
Is there an established method I should be looking at? Do I need to be providing more info with the task estimates? Further reading welcome.