Professor Frei suggests there are five distinct forms of customer-introduced variability including in which customers can differ from one another with regard to the amount of effort they are willing to apply to certain tasks, even if they are capable of performing them.

Customers demand and expect different services outcomes, even from the same service provider.  

Explanation:

There are five variabilities in the form of customer introduction.  

Thus here professor Frie suggests the variability related to customer subjective preference variability.

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