This Q and A from PEW:
PEW Ask the Expert
Q. I am always frustrated by polls asking whether one is a liberal, moderate or conservative. My feeling is that about two thirds of Americans are liberal on social issues and conservative on economic issues. (In other words they are actually Libertarians) Can’t you ask this question better? Even laying out “litmus test” questions on gun control, abortion, the effect of more or less taxes and deficits, gay marriage, national defense (foreign adventures), space exploration, size of government, global warming (and what to do about it, assuming it exists), etc. I fear that many people answer “moderate” because they are taking an average, so to speak, while having very strong but inconsistent and diverging opinions — anything but moderate.

Answer: The summary measure of political ideology you refer to has been in use — in one form or another — since the 1930s. It is useful to us for summarizing trends in ideology, and when used in conjunction with party affiliation provides a powerful way of segmenting the public. We certainly find that self-labeled conservatives tend to take conservative positions on issues, while self-described liberals tend to take liberal positions. Moderates, as you suggest, often express a mix of views.

But the question is far from perfect. For one thing, some people do not understand the terms “liberal” and “conservative.” More important, the views of some people do not fit neatly into what we have come to think of as the liberal or conservative traditions, as you suggest in your question.

In order to account for the many dimensions of public attitudes, we periodically take a look at the public through the lens of our “political typology” (see “Beyond Red vs. Blue: The Political Typology”), which uses a series of questions about basic political values to divide the public into nine core political groups. There is indeed a group of Americans — whom we labeled “Libertarians” — who express fairly liberal views on social issues and conservative views on economic issues (see the profiles of all the groups here). But this GOP-leaning group accounts for only 9% of the U.S. public. There also are two Democratic-leaning groups who hold a blend of socially conservative values and moderate-to-liberal economic values. In fact, relatively few people hew strictly to consistent liberal and conservative opinions on all issues. That doesn’t mean that the terms have completely lost their utility in American politics, but it’s one of the reasons that our approach to the study of public opinion tends to focus more heavily on questions about specific political issues than on broader questions of ideological sentiment.

Scott Keeter, Director of Survey Research, Pew Research Center
President, American Association for Public Opinion Research, 2011-2012