How do the distance of the test and control experiments comprar?

the independent variable at some normal/standard value; the part of the experiment not subjected to the dependent variable; standard of comparison for testing a hypothesis; Though the measure of distance covered while braking, record the different speeds and distances. In the case to which empirical data is compared Can refer to an experiment that block off the effect of one variable at a time or a system by holding constant all variables but the one under observation.; an experiment that parallels the main experiment except for the absence of the tested variable; standard of comparison for verifying/checking the findings of an experiment; 

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

this is more of a statement than a question. is there answers to go along with this, or is it opinion based?