Variables

If you want to make a fair experiment you are going to have to have limits or variables. If the test is not even then there will be a difference that is not what you are testing. Example: If the one shoe is heavier then the other ones then one will be easier to pull. Not because of its friction, but because of the weight.
There are two different types of variables. The dependant variables and the independent variables. The dependant variables are the measuring variable or the ones that are not chosen. The independent variables are the variables that you choose. In this case the type of shoes was the independent variables and the force was the dependant variables.