Multiplying interval variables yields meaningless results because their origin is arbitrary.

Evidence

Once you begin to have interaction terms it means that you are multiplying two variables. With ratio variables this is no issue. However, because interval variables can move around they can be both positive, both negative, or some part positive and some part negative. Individually this makes no difference, but once one multiplies it obviously does. — Gary Goertz, Social Science Concepts and Measurement, 844

Categorization

• DEFINE: Shows mechanically why the interval/ratio distinction matters in practice, not just in the abstract.