Three conditions —specifiability, verifiability, predictability— constitute an effective modular interface." #readingToday

To succeed with a nonintegrated, specialist strategy, you need to be certain you're competing in a modular world. Three conditions must be met in order for a firm to procure something from a supplier or partner, or to sell it to a customer. First, both suppliers and customers need to know what to specify—which attributes of the component are crucial to the operation of the product system, and which are not. Second, they must be able to measure those attributes so that they can verify that the specifications have been met. Third, there cannot be any poorly understood or unpredictable interdependencies across the customer– supplier interface. The customer needs to understand how the subsystem will interact with the performance of other pieces of the system so that it can be used with predictable effect. These three conditions —specifiability, verifiability , and predictability— constitute an effective modular interface.

The Innovator's Solution, Clayton Christensen