Mathematical Models

Spares Calculator optimizes spare parts using a logorithmic poisson distribution as prescribed by Dr Sherbrooke and Dr Feeney in their work for the US DoD in the 1960’s. These models are now the industry standard and are used in nearly every spare parts program on the market today. The following papers describe more about mathematical models used within Spares Calculator.

Reference papers:

  1. A System Approach to Base Stockage of Recoverable Items, G. J. Feeney and Craig C. Sherbrooke, The Rand Corporation, 1965.
  2. METRIC: A Multi-Echelon Technique for Recoverable Item Control Sherebrooke, The Rand Corporation, 1967.


Spares Calculator returns accurate results provided the input data is accurate and the equipment is in the level section of the reliability bath-tub curve. This is also known as the active life region. The equipment must not be in the infant mortality or wear out region of life and the MTBF must be stable. At this point failures are said to occur stochastically (randomly) and can be forecast using statistics. Fortunately, advances in statistical process control, reliability engineering and production screening mean that it is usually correct to assume that equipment released from quality control has a stable MTBF.


Spares Calculator was first launch in 2002. Prelaunch the Spares Calculator models were validated in thousands of trials against credible reference texts such as those listed below. Since that time the program has undergone hundreds of real-life company trials and is used globally in numerous industries by the biggest and most successful companies in the world.

Validation references:

  1. U.S. Navy, Reliability Engineering Handbook, NAVAIR 01-1A-32
  2. B L Hansen, Quality Control, Prentice Hall Inc. Englewood Cliffs
  3. U.S. Navy, Maintainability Design Criteria Handbook for Designers of Shipboard Electronic Equipment, NAVSHIPS 94324