The heuristic rule which states that where there is a large number of contributors to a result, the majority of the result is due to a minority of the contributors.. Sometimes known as the 80/20 rule) which states that, in many cases, approximately 80% of the turnover (stock etc.) can be ascribed to approximately 20% of the customers, articles or orders. The actual ratio in a particular case can be determined by ranking the customers and products etc. in order of magnitude and then calculating what percentage of the turnover (stock etc.) corresponds to 10%, 20% 30% etc. of the customer and products etc. The basis of ABC analysis.