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In that case, it must be possible to let the decision table set an attribute to unknown, using the symbol * or ? to achieve this.
(undoubtedly the symbol ? would have been a better candidate for that).

See the example below:

The example above should be read as follows: For males, the discount for some fictitious product is 5%, 7% or 10%, depending on the age of the applicant. For females over 18 the percentage is 5%. For females aged 18 or younger the discount cannot be inferred, a case handler should determine the discount on a case-by-case basis.

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