And sometimes, political knowledge is treated as a component for some other measure.
This is a story of the latter, a paper I stumbled across that examines religiosity, political ideology, and political engagement. Formally, the title is The Association of Religiosity and Political Conservatism: The Role of Political Engagement. You can find a PDF of it here. The study is an example of using political knowledge as part of something else. On page 11 of the paper, the authors tell us:
We conceptualize political engagement as overall involvement with political information, as manifested by a) high (vs. low) subjective importance of politics and b) high (vs. low) objective political knowledge ... These indicators tend to be correlated, but they are conceptually distinguishable.
In other words, political knowledge is like political interest, only different, but together they make for something called engagement. I could quibble with this (engagement, after all, should include behavior such as news consumption, perhaps participation), but my point today is knowledge can be used in creative ways other than merely as an independent or dependent variable.