What is single fault assumption w.r.t boundary value analysis?

In the book How we test software at Microsoft, the author(s) make the below comment. Appreciate if someone can explain

basic boundary testing is based on the single-fault assumption. With this underlying assumption, BVA testing is generally not effective in evaluating complex combinations of dependent or se开发者_开发百科micoupled parameters.

1. What is single fault assumption
2. And why is basic boundary testing based on single fault assumption?

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The single fault assumption is the assumption that failures are only rarely the result of the simultaneous occurrence of two (or more) faults.

The reason basic boundary testing is affected by this assumption is because this technique will create different testing scenarios by focusing only the boundaries of a single data input, disregarding the combination effect of multiple inputs.

This is the main assumption of the technique, in my mind it is also the reason why it is so simple and easy to apply.

Hope this helps!

-joel