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.
In your case, the parameter of interest is the difference of means: $\mu_x-\mu_y$. If you construct a one sided confidence interval for this parameter, of the form $(a,\infty)$, then you can say with 95% confidence that $a<\mu_x-\mu_y$. Thus, if $0\leq a$, you may reject the null hypothesis.