Traditionally I've always stuck to the number 10. I recall reading some pretty convincing research that said 10 is the point at which 90% of problems will have been discovered and the gain from adding more users begins to level off.
Today I go to search for an article on this so I can include it as a reference as to why we used 10 people in our test though...and I find Nielsen is now saying 5 is the number. And apparently has been for a decade. Total case of mis-remembering on my part?
But I search further...and find an article saying 20 is the perfect number if you can manage it!
And then here's one that says 15!
Apparently everything but the 10 that has been my standard for years and has got me through life thus far...I guess I always spoke convincingly enough that people just took what I said as a given.
So many conflicting numbers. Which is right?