Click here to purchase the entire book in PDF format. Variables When you're designing a listening test, you have two types of variables to worry about independent variables and dependent variables. What's the difference? An independent variable is the thing that you (the experimenter) are changing during the test. For example, let's say that you are testing how people rate the quality of MP3 encoded sounds. You want to find out whether one bitrate sounds better than another, so you take a single original sound file and you encode it at different bitrates. The test subject is then asked to compare one sound (with one bitrate) against another (with a different bitrate). The independent variable in this case is the bitrate of the signal. Note that it's independent because it has nothing to do with the subject - the bitrate would be the same whether the subject listened to the sounds or not. A dependent variable is the thing that you're measuring in the subject's responses. In the example test in the previous paragraph, the dependent variable is the subjects' judgment of the quality of the sound. The variable is dependent on the subject - if the subject never hears the sounds, they can't make a judgment on the quality. Here's another example. Do a test where you have people taste two cups of coffee. One cup of coffee has no sugar in it. The other cup has 2 spoonfuls of sugar. You ask the people to rate the sweetness of each cup of coffee. In this test, the amount of sugar in each cup is the independent variable. The perception of sweetness is the dependent variable.