How do people measure the degree of democracy?

You have probably heard about people saying that democracy in America is declining and then showing numbers to back this up. What I would like to know is how these numbers are computed. Is there some document that says what factors are computed and how they are combined? There are probably a combination of qualitative and quantitative factors, which is fine, but what is the formula?

For example, I checked the Varieties of Democracy Web site over here and did not find much in the way of specifics.