In physics, we use 2-sigma (95%) limits all the time. 5-sigma (99.9999%) is generally required only for a claim of detection. If you're just feeling your way a

In physics, we use 2-sigma (95%) limits all the time. 5-sigma (99.9999%) is generally required only for a claim of detection. If you're just feeling your way around in the dark, 2-sigma is a useful way to work, so we use that to guide exploration. Why 2-sigma? Well, it's twice as big as 1-sigma. Experiment didn't go well, but you need a more-impressive result? Use a 90% confidence interval instead of 95%.