​Observe an experiment with repeated trials.

​Mrs. Garza’s class was divided into four groups of four students in each group. The four groups were named Red, Blue, Purple, and Orange. Each group was given six sets of the same materials. Each set contained: 1 gram of wheatgrass seeds, 1 four‐inch plant pot, and 200 grams of potting mix. The class planted a total of 24 pots of wheatgrass plants.

​The 24 plants were divided into two groups: twelve of the plants were placed on the counter and received full sunlight every day. The other twelve plants were placed in an empty cupboard with the door closed. The door was only opened to water the plants and measure the height of each plant. Data was recorded for ten days.

​Mr. Williams class had the same number of students and was grouped the same way. Group names were the same as Mrs. Garza’s class. Mr. Williams did a teacher demonstration with the same materials, but only planted two plants. One plant was placed in on the counter in the sunlight, and one was placed in a cupboard with the door closed. Data was recorded for ten days.

​In both classrooms, an air conditioning vent blew cold air onto one side of the counter. Mrs. Garza’s twelve plants were in a line on the counter. Three of the plants were directly under the vent. Mr. Williams one plant was directly under the vent. After ten days, it was observed in Mrs. Garza’s class that the three plants under the vent didn’t grow. Turned out to be ok, because they still had nine plants that grew just fine. Their investigation was a success.

​Mr. Williams investigation failed. His one plant was placed directly under the AC vent, and did not grow.

​One of the reasons for experiments to have repeated trials is to account for variation: not all twelve plants will grow at exactly the same rate, but across twelve plants there is enough data to say whether or not plants grow better in sunlight or dark. Another reason to have repeated trials of an experiment is to make sure the experiment is right: without repeated trials (only one plant...) something can happen to ruin the investigation.

​Repeated trials make sure that the results of an investigation are reliable. Mrs. Garza’s class concluded that plants grow better in the sunlight, because they had enough evidence to justify that claim. Mr. Williams class concluded that plants grow better in the dark, because that’s what their data showed. The problem is, their data wasn’t reliable, because it was based on only one plant that died in the cold air from the AC.