Fair comment: Books and other products sold by online retailers can attract thousands of reviews. Why are they worth reading — or writing? The sheer volume of reviews makes far more difference, according to Google’s analysis of clicks and sales referrals. “Single digits didn’t seem to move the needle at all,” says Mr McAteer. “It wasn’t enough to get people comfortable with making that purchase decision.” But after about 20 reviews of a product are posted, “We start to see more reviews—it starts to accelerate,” says Sam Decker, the chief marketing officer of Bazaarvoice, a firm that powers review systems for online retailers. [...]What is true for reviews does not appear to apply to comments left on news stories or blogs, however. “You can probably have a decent discussion until you get to about 350 comments,” says Markos Moulitsas, the founder of Daily Kos, a popular left-leaning political site. But after that, he says, “most outside people will stay away from the thread, and further growth will come from people already inside that thread carrying forth a discussion, debate, or argument.”
