Before you accept a result, ask one question: compared with what? A claim needs a comparison group, which is treated the same way except for the one thing being tested.
This lesson starts the set on evaluating a science claim from a supplied dataset. It connects to identifying variables and fair comparisons.
What is the claim with one group?
An original classroom report says, “We gave 6 bean plants plant food for 14 days, and they reached an average height of 18 cm. Plant food makes beans grow tall.”
The number sounds convincing, but 18 cm means little on its own. Bean plants grow taller over 14 days even without plant food, so the report does not show what the food added.
Worked example: adding the comparison group
A second version of the report includes the missing group.
| Group | Treatment | Plants | Average height after 14 days |
|---|---|---|---|
| Test | Plant food, 5 mL every 2 days | 6 | 18 cm |
| Comparison | Water only, same amount | 6 | 15 cm |
All 12 plants used the same bean seeds, the same pot size and soil, and the same window.
Step 1: find the comparison. The test group is 18 cm and the comparison group is 15 cm.
Step 2: find the difference. The difference is 18 − 15 = 3 cm.
Step 3: state what the data supports. In this test, plants given plant food grew 3 cm taller than plants given water only.
Step 4: state a limit. Six plants per group and one bean type cannot show the same result for other plants. The next lessons discuss whether 3 cm is large enough to matter.
What must stay the same?
Think of everything that could also change the height: seed type, pot size, soil, water volume, light and temperature. The comparison group must match the test group on each one, so the only difference is the plant food.
The mistake that costs marks
The slip is to draw a conclusion from one group, or to compare with a group that differs in more than one way.
| Wrong | Right |
|---|---|
| “The plants reached 18 cm, so plant food works” | “There is no comparison group, so the claim is not supported” |
| “The food group grew more than plants in another class” | “The groups were kept in different places, so the comparison is unfair” |
| “Plant food is needed” | “With plant food the plants were 3 cm taller in this test” |
The repair is to write the comparison group in your answer, with its figure, before any conclusion.
Check yourself
A fictional report says, “After drinking a sports drink, 8 runners ran 400 m in an average of 75 s, so the drink makes you faster.” What comparison group is needed, and what should be kept the same?
Answer
A second group of 8 runners who run the same 400 m after drinking water only (or nothing) is needed. Without it, 75 s has nothing to be compared with.
Keep the same: the runners’ ability (for example, the same runners in both trials on different days, or matched groups), the track, the time of day, the weather and the rest time before the run. Only the drink should differ.
What to study next
Next, explaining why a small difference may not support a strong conclusion checks whether a gap like 3 cm is large enough. The graph evidence and fair-comparison lab offers more supplied data.
To practise with a teacher, see online one-to-one Science tuition.