A heuristic I use when looking at products with low numbers of reviews is to add one positive and one negative review, so (positive+1)/(positive+negative+2).

A heuristic I use when looking at products with low numbers of reviews is to add one positive and one negative review, so (positive+1)/(positive+negative+2). This basically makes the 'default' rating 50% or 3 stars or whatever, and votes move the rating from that default.