Variables, hypotheses, sampling and tools

A research question becomes researchable when its ideas are turned into variables that can be observed and measured. Does a new teaching method improve learning? The method is the independent variable, learning is the dependent variable, and prior ability is an extraneous variable that must be controlled. Worked example: if the new-method class also has the best teacher, teacher quality — not the method — may explain the result; that is an extraneous variable left uncontrolled.