How much of an impact do reviews and ratings have on online sales? Does the number of reviews and ratings matter? Are reviews and ratings more important for higher-priced products/services?

Northwestern University's Spiegel Research Center and PowerReviews recently tackled those questions in an in-depth research study.

The researchers examined detailed data from two online retailers of primarily lower-priced product categories and a third online retailer of higher-priced gifts to determine the relationship between reviews/ratings and purchase behavior (see the full report for the complete methodology).

Including reviews on product pages measurably boosts sales, and conversion rates steadily rise with the first few pieces of feedback, the analysis found.

The conversion rate on a page that includes five reviews is 270% higher, on average, than that of a page with no reviews.

The benefits of including more reviews diminish quickly once a base number is reached, the analysis found.

For one of the e-commerce sites examined, nearly all of the increase in purchase likelihood came from the first 10 reviews, and the first 5 reviews drove the bulk of the increase.

The impact of reviews on sales is especially strong with higher-priced products: For the gift retailer included in the study, the conversion rate increased 190% when reviews were displayed for a lower-priced product but spiked 380% when reviews were displayed for a higher-priced product.

Star ratings also have a measurable impact on sales, the analysis found.

However, the highest-rated (5 stars out of 5) products do not necessarily have the highest lift in conversions.

Purchase likelihood typically peaks for products with ratings in the 4.0 - 4.7 range, and then begins to decrease as ratings approach 5 stars.

The sales lift from ratings varies significantly by product category: Some items see a steady increase in purchase probability as ratings increase ,while other items see a more uneven impact.

About the research: The report was based data from two online retailers of primarily lower-priced product categories and a third online retailer of higher-priced gifts.