Techniques useful in the planning phase of productive systems and elsewhere are presented. Assuming only that item distributions of times to failure and repair are exponential, the availability of any redundant stage consisting of identical items can be calculated in terms of the item availability, without specific knowledge of failure and repair rates. The availability of a series of stages is greater than the product of the stage availabilities, if unfailed stages are not at risk during system failure. A general expression is developed to account for this. By considering all the possible states of the stages of a system, the overall productiveness or system effectiveness can be estimated. Bathtub curve, maintenance and interstage storage effects are considered in general terms only.