How does the design of your experiment control for outside factors that may affect the results?

A factor of an experiment is a controlled independent variable; a variable whose levels are set by the experimenter. A factor is a general type or category of treatments. Different treatments constitute different levels of a factor

An experiment has several types of variables, including a control variable (sometimes called a controlled variable) ... A control variable is another factor in an experiment; it must be held constant. In the plant growth experiment, this may be factors like water and fertilizer levels.