in a memo to his staff last month. Read the memo, it's actually quite clear in how a specific article was biased. The second to the last paragraph spells it out clearly:

The reason I'm sending this note to all section editors is that I want everyone to understand how serious I am about purging all political bias from our coverage. We may happen to live in a political atmosphere that is suffused with liberal values (and is unreflective of the nation as a whole), but we are not going to push a liberal agenda in the news pages of the Times.

He's admitting that a liberal bias exists and, to his credit, is trying to get rid of it. Funny how you're unable to admit that bias exists.