It seems obvious, but deserves emphasis: consistent outcomes can only be achieved if there is control of:

• An adequate requirements set to characterize the desired outcomes and the acceptable limits of variation
• The processes used to produce the desired outcome (and their fitness with respect to these requirements)
• A means of validation of the suitability of the outcome from the perspective of the customer, to control and improve the requirements and processes
• Utilization of actors who have sufficient training, understanding, and resources to fufill their roles in these activities
• Enough improvement cycles to remove common-mode errors and drive first-time quality into the outcomes and means of production

People often believe they are performing consistently, even though independent assesments determine that they are not. This is a result of the many different biases inherent in measuring human performance. Training and communities of practice are typically used to deal with such cognitive biases, as well as variation in individual competencies, and thus assure process quality and adherence over time.