Original Project
Control of Variables
If more than one variable changes at the same time in an experiment, the outcome of the experiment may not be cl…
Item CR020003: In an experiment on the effect of temperature and the amount of water on the growth of plants, with both variables changing at the same time, no conclusion can be reached regarding the effect of either amount of water or termperature on the growth of the plants.
Students are planning to grow plants from seeds. They want to find out which of two temperatures, 60°F or 90°F, is better for growing these plants. They also want to find out if one cup of water or two cups of water is better for growing these plants.
They do the following experiment. They use two trays with identical soil and they plant ten seeds in each tray. They keep Tray X at 90°F and Tray Y at 60°F. They use two cups of water for Tray X and one cup of water for Tray Y. After a few days, they count how many plants are growing in each tray.
What can the students conclude from this experiment?
- They can conclude that 60°F is better than 90°F for growing these plants.
- They can conclude that one cup of water is better than two cups of water for growing these plants.
- They can conclude that 60°F is better than 90°F for growing these plants and that one cup of water is better than two cups of water for growing these plants.
- It is not possible to conclude from this experiment if 60°F is better than 90°F for growing these plants or if one cup of water is better than two cups of water for growing these plants.
- Distribution of Responses
- Students Responding Correctly
| Group | Correct | Total | Percent |
|---|---|---|---|
| Overall | 735 | 2686 | 27% |
| Grades | |||
| 6–8 | 327 | 1506 | 22% |
| 9–12 | 402 | 1166 | 34% |
| Gender | |||
| Male | 378 | 1323 | 29% |
| Female | 351 | 1339 | 26% |
| Primary Language | |||
| English | 665 | 2267 | 29% |
| Other | 63 | 376 | 17% |