An experimental investigation is an organized process used to test an idea. In a good experimental investigation, one must eliminate error to ensure that the results are due to the factor or factors being tested.
Let’s look at an experiment with plants. Kaitlin is trying to determine how pH affects the germination rate (time it takes for the seeds to sprout). Click through the following animation to see how the experiment was set up.
Was Kaitlin’s conclusion correct? Can we make any inferences based on her experiment?
Remember that an experimental investigation is a “fair test” designed to gather evidence to support or refute a causal relationship. Can we tell what caused the results of Kaitlin’s experiment? The answer is no. Kaitlin did too many things differently in the experiment. Each cup was a different size and there was a different amount of soil in each cup. She planted varying amounts of beans in the cups and each cup received a different amount of water.
Let’s compare the two types of variables. Click on each variable to learn more about each type of variable.
Let’s look back at Kaitlin’s experiment.
She covered the beans with soil and added water to the cups. The water had been treated with chemicals to alter the pH. She added the following amounts of the different pH solutions to the cups.
- 3 ounce cup—3 mL of water solution with a pH of 3
- 10 ounce cup—10 mL of water solution with a pH 7 (neutral)
- 16 ounce cup—16 mL of water solution with a pH of 9
She changed too many variables. In an experimental investigation there should only be one independent variable tested at a time!
What should have been Kaitlin’s independent variable? (Remember she was trying to determine how pH affects the germination rate (time it takes for the seeds to sprout).