Consumers have always valued opinions expressed directly to them. Marketers may spend millions of dollars on elaborately conceived advertising campaigns, yet often what really makes up a consumer’s mind is not only simple but also free: a word-of-mouth recommendation from a trusted source. As consumers overwhelmed by product choices tune out the ever-growing barrage of traditional marketing, word of mouth cuts through the noise quickly and effectively.

Indeed, word of mouth is the primary factor behind 20 to 50 percent of all purchasing decisions. Its influence is greatest when consumers are buying a product for the first time or when products are relatively expensive, factors that tend to make people conduct more research, seek more opinions, and deliberate longer than they otherwise would. And its influence will probably grow: the digital revolution has amplified and accelerated its reach to the point where word of mouth is no longer an act of intimate, one-on-one communication. Today, it also operates on a one-to-many basis: product reviews are posted online and opinions disseminated through social networks. Some customers even create Web sites or blogs to praise or punish brands.

The full article (registration required) looks at what it calls WoM 'equity', an index of the brand's power to create messages that influence the consumer to buy. The primary driver of WoM impact is the content of the message, the second is the identity of the person who conveys the message and the third is the environment where WoM circulates.

Source: McKinsey Quarterly, Number 2, 2010