science fair test !

I do not know exactly what you mean,so I just guess what you are asking.
Fair tests are those tests which have only one variable condition while other conditions are kept the same. Unfair tests are those tests which have more than one variable condition so comparison between the samples is meaningless.
FOR example if you want to compare the rate of photosynthesis of two different kinds of plants, you should ensure that the plants receive same amount of sunlight water carbon dioxide and other conditions this will then be a fair test. However,if one of the plant receive more sunlight, the rate of photosynthesis will be affected ,there is one more varIable(sunlight) so this will be an unfair test.