The state of being expert; skill or proficiency(noun)
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Use "expertness" in a sentence
"Regarding the subject at hand here, though, I think that "expertness" in the case of the violinist and the programmer, it had a lot to do with being an expert at the use of the very few tools for those 10,000 hours (violin + sheet music or computer and programming language)."
"I was ASTOUNDED by Mexico's intelligence and expertness in response and preparation for the cane."
"Paraprofessional aides are much less technically expert than are professionals, but they often more than make up for their lower degree of technical expertise through their greater social expertness."