The proper use of a differences test for two variables' averages requires that you select two metric variables that are measured with the same scale units.

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This statement is true. In order to compare the averages of two variables using a differences test (e.g. t-test), the variables must be metric, meaning that they are measured using interval or ratio scales, and they must have the same units of measurement. For example, you could compare the average height of men and women in inches, or the average income of two different countries in dollars. However, you couldn't compare the average height of men and the average temperature in Celsius, because temperature is a different type of measurement and has different units.

The Proper Use of a Differences Test for Two Variables