List some variables where accurate measurement is difficult.
Variables where accurate measurement is difficult include fairness, honesty, and beauty.
These variables cannot be measured in the same way as we measure income, weight, or height. At most, these things can be measured relatively, for example, we may be able to rank people according to beauty. However, even ranking is debated because standards and criteria may differ from person to person and culture to culture.
When we wish to find the relation between variables where at least one is of this type, Spearman's rank correlation coefficient is used instead of simple correlation coefficient.
Explanation
The textbook explicitly mentions fairness, honesty, and beauty as examples of variables that cannot be measured accurately like income, weight, or height. These are qualitative variables where measurement is subjective and relative at best. The context explains that even ranking such variables is debatable due to differing standards across persons and cultures. This is why Spearman's rank correlation is preferred when dealing with such variables.