Ries identifies three primary engines of growth: sticky, viral, and paid. Each engine has its own set of metrics that determine how fast a company can grow. For instance, the viral engine of growth relies on person-to-person transmission as a necessary consequence of normal product use. The viral coefficient measures how many new customers will use a product as a consequence of each new customer who signs up. A viral coefficient greater than 1.0 indicates exponential growth. 'Sustainable growth is characterized by one simple rule: New customers come from the actions of past customers,' Ries explains. | Eric Ries | The Lean Startup

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Engines of Growth

Ries identifies three primary engines of growth: sticky, viral, and paid. Each engine has its own set of metrics that determine how fast a company can grow. For instance, the viral engine of growth relies on person-to-person transmission as a necessary consequence of normal product use. The viral coefficient measures how many new customers will use a product as a consequence of each new customer who signs up. A viral coefficient greater than 1.0 indicates exponential growth. 'Sustainable growth is characterized by one simple rule: New customers come from the actions of past customers,' Ries explains.

-- Eric Ries