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Proficient in is the most popular phrase on the web.

Proficient at 1193,000 results on the web	Proficient in 118,200,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Neither of us was proficient at using a saw and we'd managed only two rungs when exhaustion caught up with us and the light faded for permanently. While the ... He has become very proficient at computer programming. She is proficient in two foreign languages. With this system, a large pool of teachers are asked to ... May 20, 2008 ... I always find "proficient in a language" and "skilled in a language" but at times I find examples with "proficient at doing" and "skilled at doing" Jan 11, 2012 ... If you meant he was skilled in understanding and working with English people, you'd have to say "proficient at working with English people".	Are you proficient in other languages? Proficient in folk dancing I see. cha. I'm not proficient in that sort of... He's proficient in six different martial arts, and he lives most of the year in Brussels. Proficient in the rumba, waltz and cha He fought someone proficient in martial arts. He'd be proficient in sewing skins and hides.

Proficient at

1193,000 results on the web

Proficient in

118,200,000 results on the web

More popular!

Neither of us was proficient at using a saw and we'd managed only two rungs when exhaustion caught up with us and the light faded for permanently. While the ...
He has become very proficient at computer programming. She is proficient in two foreign languages. With this system, a large pool of teachers are asked to ...
May 20, 2008 ... I always find "proficient in a language" and "skilled in a language" but at times I find examples with "proficient at doing" and "skilled at doing"
Jan 11, 2012 ... If you meant he was skilled in understanding and working with English people, you'd have to say "proficient at working with English people".

Are you proficient in other languages?
Proficient in folk dancing I see.
cha.
I'm not proficient in that sort of...
He's proficient in six different martial arts, and he lives most of the year in Brussels.
Proficient in the rumba, waltz and cha
He fought someone proficient in martial arts.
He'd be proficient in sewing skins and hides.