When two measurements change together, you have an association. A cause needs more: a controlled test, or strong reasoning that other factors do not explain the pattern.
This lesson is part of ecological evidence and population relationships. The habit also appears in avoiding unsupported environmental claims.
What does a survey show?
A field survey records what is already there. It can show that two factors vary together, but it cannot control them, so other differences between sites stay hidden in the data.
Worked example: a river survey
The data is invented. A student surveys 6 sites along a stream. The sites with higher dissolved oxygen have more fish.
| Site | Oxygen (mg per litre) | Fish counted | Water temperature (°C) |
|---|---|---|---|
| 1 | 4 | 3 | 31 |
| 2 | 5 | 5 | 30 |
| 3 | 6 | 8 | 28 |
| 4 | 7 | 10 | 27 |
| 5 | 8 | 12 | 26 |
| 6 | 9 | 13 | 25 |
Oxygen and fish rise together, which is an association. But the temperature falls in the same order, and cooler water holds more oxygen. So oxygen, temperature or both may relate to the fish counts.
A safe conclusion is: “More fish were found at sites with higher oxygen, which suggests a relationship, but temperature also differs between sites, so a cause is not shown.”
What is the three-question test?
- Did the investigation change one factor and keep the others the same?
- Were there repeats, and does the result stay the same?
- Can another factor explain the pattern?
If the answer to question 1 is no, or the answer to question 3 is yes, write “associated with” and name the other factor.
The mistake that costs marks
The common slip is “low oxygen causes fish to die” from the table alone. The data does not test that.
To test for a cause, design a controlled experiment, for example keeping fish in tanks with different oxygen levels at the same temperature, with repeats.
Check yourself
A survey finds more nests in areas with taller trees. A student writes “tall trees cause more nests.” Rewrite the conclusion and name one other factor.
Answer
“Areas with taller trees had more nests, which suggests an association, but it does not show that height is the cause.”
One other factor: tall trees may also offer more food or shelter, or less disturbance, and any of these could affect nesting. Naming one is enough.
What to study next
Try the practice set for this cluster. The biological data interpretation trainer has more survey tables.
For teacher support, see online one-to-one Biology tuition.