All interval data assumes equal divisions between data points. Note how the examples above do that. But, IQ tests and many other scales lose that property at their extremes. With interval data you may perform a wide variety of statistical tests including means, deviations, correlations, and tests of significance. Ratio Ratio data have equal distances between units and an absolute zero point that is not arbitrary.
Experimenters select their IVs, hence their name.
IVs are also known as levels or treatments. Whenever you think of a DV, think of a way of measuring the IV in your research project. DVs depend on the IV, picking a DV is akin to an art, and the more you do it, the better you will get at it. Every other possible variable is an extraneous variable. In experimental design, experimenters strive to reduce or measure the effects of extraneous variables.