Median real estate property taxes paid for housing units with mortgages in 2013: $590 (1.2%). Median real estate property taxes paid for housing units with no mortgage in 2013: $637 (1.5%). Nearest city with pop. 50,000+: Bismarck, ND (78.6 miles, pop. 55,532). Nearest city with pop. 200,000+: Minneapolis, MN (305.7 miles, pop. 382,618).
The third type of variable is a controlled variable. This variable is constant in an experiment. Experiments are repeated a number of times to interpret how a change in the independent variable affects each trial result.
Variables are measured along some type of underlying dimension or scale. Sometimes the dimension is simple - as in present vs. absent, or yes vs. no. This section provides a finer gradation of levels than what you have learned with regard to categorical versus continuous variables.