Stafford Beer developed a Productivity Index 
.
Done in a factory setting.
Actuality -- What did we do today?
Capability -- What is the best we have ever done. Where everything went right?
Potentiality -- What could we be doing?
Latency -- The ratio of capability and potentiality.
What could we do if we made some relatively minor modifications? (Bottle necks, missing information, ..)
What could be done to make big improvements with small changes?
Ratios
Actuality:Potentiality = Performance (the final ratio)
Stafford Beer's Capacity Ratio Measures.
Three Measures of Capacity (Actuality, Capability, Potentiality) each made on the same metric.
Three ratio indices Measures of Achievement (Productivity, Latency, Performance) where unity is the maximum measure.
Productivity as 'Special Cause Variation'.
Latency as 'Common Cause Variation'.
See Common and Special Cause variation, Wikipedia
Note: Weibull distribution, Wikipedia
Bayes Theorem, Wikipedia for use when human beliefs alter probabilities.