A fair experiment is a test designed so that the evidence answers one clear question. It matters because good experimental design helps students tell the difference between a real effect and a result caused by bias, chance, or sloppy procedure. In science writing and ELA, explaining a fair experiment also means using precise claims, evidence, and reasoning.
A strong plan makes the final conclusion more believable.
Key Facts
- A testable question names what will be changed and what will be measured.
- Hypothesis format: If the independent variable changes, then the dependent variable will change because of a scientific reason.
- Independent variable = the one factor the experimenter deliberately changes.
- Dependent variable = the outcome measured in response to the independent variable.
- Controlled variables = all other important conditions kept the same to make the test fair.
- More trials and larger sample sizes reduce the effect of random error and make results more reliable.
Vocabulary
- Hypothesis
- A hypothesis is a testable prediction that explains what you think will happen and why.
- Independent Variable
- The independent variable is the factor the experimenter changes on purpose.
- Dependent Variable
- The dependent variable is the result or outcome that is measured.
- Controlled Variable
- A controlled variable is a factor kept the same so it does not affect the results.
- Control Group
- A control group is a comparison group that does not receive the experimental change.
Common Mistakes to Avoid
- Changing more than one independent variable at the same time is wrong because you cannot tell which change caused the result.
- Measuring the dependent variable in different ways is wrong because inconsistent measurements make the data unfair to compare.
- Using too few trials is wrong because one unusual result can strongly affect the conclusion.
- Forgetting a control group is wrong when a comparison is needed because there is no baseline for judging whether the treatment made a difference.
Practice Questions
- 1 A student tests whether fertilizer affects plant growth. Group A gets no fertilizer and Group B gets fertilizer. Each group has 12 plants. What is the independent variable, the dependent variable, and the sample size?
- 2 A class runs 5 trials for each of 4 ramp heights to test how ramp height affects toy car speed. How many total trials are completed, and what variable should be kept the same besides ramp height?
- 3 A student claims that a new study method improves test scores, but the students choose whether to join the study group or the regular group. Explain why random assignment would make this experiment fairer.