When conducting an experiment, why is it important to test only one variable at a time?

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A variable within a scientific experiment is something that is changed (i.e. it varies). There should be only one  manipulated variable within a scientific experiment so that the experimenter can be certain it is this variable which causes a pattern in the resultant data, if any exists at all. If there were multiple variables, then the experimenter would not know what accounted for the results of the experiment. 

For example, say an experimenter was testing the respiration rate of goldfish. He had three goldfish each in five different tanks. He increased the salt concentration in each tank by one tablespoon. He increased the temperature within each tank by intervals of ten degrees. All the fish in tank five die. Becuase the scientist changed more than one variable, he is not able to determine if the death of the fish was due to the high salt concentration or high temperature. 

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