When scientists form a conclusion about a relationship, they need to be sure that their conclusion is valid or correct. Other scientists will accept the results of an experiment only if the conclusion is valid. So, how can scientists decide whether their conclusions are valid? One way is to look at the design of the experiment. The conclusion drawn from their data was valid. Next, consider the experiment done by teams who reported their data in Table 3. Those teams also changed the size of the magnet. But they changed another variable too. They changed the material from which the magnet was made. So, the Table 3 teams could not be sure whether any effect they saw was caused by the different sizes of the magnets or was caused by the different materials from which the magnets were made or was caused by both variables. The experiment they performed was not a fair test, and the conclusion drawn from their data was not valid. For an experiment to be a fair test, the only two variables that can change are those for which you are trying to determine a relationship. All other variables must be kept the same. The procedures for the experiment must also stay the same. If this is not the case, the experiment is not a fair test and you cannot form a valid conclusion.
Now let’s consider another example. In Activity 2, you did an experiment to answer the question: “If the length of a pendulum increases, what happens to the time it takes to make 10 back and forth swings?”
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Participate in a class discussion to review the conclusions for the teams who recorded their data in Table 2 or Table 3. 
