Pablo and Johanna have to do a yearlong study for their biology course. After some discussion, they decide to try comparing their dogs and the diet that they feed them to test their hypothesis that the local veterinarians special dog food mix will enhance growth and development. Each student adopts a puppy from the local pound. Pablo plans to feed his goldendoodle the special diet, while Johanna plans to use generic dry kibble from the supermarket for her bulldog. Pablo and Johanna appear not to have given consideration to the importance of controlling for ________.

The question above presents a scientific experiment carried out by students Pablo and Johanna from the biology course. In this experiment, they both compare the results of different diets on their dogs. Up to this point, the experiment was being established correctly, since the variables independent (the composition of the dogs' diet) and dependent (the influence of the diet on the dog) were defined. However, these students made a mistake in using dogs of different breeds.

In an experiment, everything must be very controlled and the treatments must be exactly the same. In the above case, the dogs are the treatments and the fact that they belong to different breeds, means that the results of the experiments can present significant differences, not providing a correct conclusion of the experiment.