Second black box testing technique is boundary value analysis. It is extension of equivalent class partitioning

Boundary is where maximum defects are found

So as in example where an input field accepts values between 20 and 60 the boundaries are 19,20,60,61

Another example.

Let us say a website for kids only accepts age more than 10.

In that case we can make 3 test cases based on test data.

For first one we test 9 which should fail.

Second is 10 which should also fail

Last is 11 which should pass.

Another thing is boundary value analysis can check numerical data and alphabets alike.

Let us say there is a username field on a registration field which accepts a max of 15 characters.

So using BVA we can check two values 15 characters which should pass and 16 characters which should fail.