Although many metrics explore brand loyalty, this one has been called the “acid test.” It can tell a company much about the attitude of its customers and whether its position in the market is likely to be defensible against sustained pressure from a competitor. 
The purpose of the metric is to assess the commitment of a firm’s or a brand’s customer base. Brand or customer loyalty is a key marketing asset. Marketers evaluate aspects of it through a number of metrics, including repurchase rate, share of requirements, willingness to pay a price premium, and other AAU measures.
Perhaps the most fundamental test of loyalty, however, can be captured in a simple question:
When faced with a situation in which a brand is not available, will its customers search further or substitute the best available option?
Willingness to search (%) = Percentage of customers who are willing to leave a store without a product if their favorite brand is unavailable
- ^ Farris, Paul W.; Neil T. Bendle; Phillip E. Pfeifer; and David J. Reibstein (2010). Marketing Metrics: The Definitive Guide to Measuring Marketing Performance (Second Edition). Upper Saddle River, New Jersey: Pearson Education, Inc.