The Common Wisdom Says It's Price. We define an impulse purchase as a spontaneous purchase - an item that a shopper hadn't planned to buy when they began their shopping task. Both The Yankee Group and Ernst & Young conducted surveys where they asked people why they would make impulse purchases on the web. According to The Yankee Group (November, 2000), 75% of survey respondents indicated that a "special sale price" would motivate them to make a spontaneous purchase. The second most influential factor was free shipping (49% of respondents). Ernst & Young (January, 2000) reported that 88% of impulse purchases were because shoppers found products that were offered at good price [or] on sale. According to these surveys the common wisdom is clear: impulse buys are price-related and not due to any specific design or architecture of the web site.
Not so very long ago, it was agreed that five to eight users was enough for a good usability test. Somehow, this idea achieved mythic status. We believed it. We preached it to everyone who would listen. It survived in areas where it had been disproved, and was introduced into new situations where it didn't even apply.