To compare a conclusion with an alternative, state the conclusion, name a rival explanation that could give the same pattern, then use the method and the data to say which is better supported. Good answers stay tied to numbers.
This lesson is part of investigations and evidence and uses the idea of controlled conditions. The data is invented.
How do I build the comparison?
- State the pattern in the data, with numbers.
- State the proposed conclusion.
- Name one alternative cause and say why it fits the data.
- Say what the method does or does not rule out.
- Say what extra evidence would separate them.
Worked example
Five plants grew in a bright spot and five in a shaded spot for two weeks. Invented growth in cm:
| Spot | Growth of each plant (cm) | Mean (cm) |
|---|---|---|
| Bright | 3.9, 4.4, 4.1, 4.3, 4.3 | 4.2 |
| Shaded | 2.6, 3.0, 2.8, 2.9, 2.7 | 2.8 |
Step 1, check the means. Bright: 3.9 + 4.4 + 4.1 + 4.3 + 4.3 = 21.0, and 21.0 ÷ 5 = 4.2. Shaded: 2.6 + 3.0 + 2.8 + 2.9 + 2.7 = 14.0, and 14.0 ÷ 5 = 2.8.
Step 2, state the pattern. The bright plants grew 4.2 − 2.8 = 1.4 cm more on average, and the ranges (3.9 to 4.4 and 2.6 to 3.0) do not overlap.
Step 3, conclusion. Light level affected growth.
Step 4, alternative. The bright spot was nearer a window, so it may also have been warmer. Temperature then explains the same pattern.
Step 5, judge. Temperature was not recorded, so it is not ruled out. The data are consistent with the conclusion but do not exclude the rival. Recording temperature, or placing both sets of plants where it is the same, would separate them.
The mistake to watch for
Mistaken answer: “The alternative is that the experiment was unfair.”
That is a general complaint with no cause named. Give a specific rival factor, show how it could produce the pattern, and say what evidence would test it.
Check yourself
1. Give the difference in means between bright and shaded plants.
Show answer
4.2 − 2.8 = 1.4 cm.
2. Rewrite “light proves growth” with stronger wording.
Show answer
“The data are consistent with light increasing growth, although temperature was not controlled.”
3. What single extra measurement would help separate light from temperature?
Show answer
Record the temperature at each spot, or keep it the same for both.
Where this leads next
The last lesson is about where your own practical work happens: separate examination preparation from practical-centre provision. Then use the investigations practice set.
A teacher in online one-to-one Combined Science tuition can check that your alternatives are specific and tied to the data.