Experiment to Test the Time Neutrase and Milk Solution Takes to Clear

What other factors may affect the results?
I shall have to make sure that none of the solutions become contaminated as this would affect the experiment. One factor that will always be difficult to compete with is when to judge that the solution is clear, one person's interpretation of clear will be different to another person's. So I will have to make sure that I judge the clearness fairly for each test.
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