A controlled condition is a factor kept the same in every trial, so it cannot be the reason the results differ. In Combined Science, data-handling and planning questions ask you to name the controlled conditions and to notice when one was not actually controlled.
This lesson belongs to investigations and evidence. The data below is invented for practice.
How do I tell what was kept the same?
Start with three labels: the independent variable (what is changed on purpose), the dependent variable (what is measured) and the controlled conditions (everything else that could matter). Then read each row of the results and check that every controlled condition has the same value.
If two things differ between rows, you cannot say which one caused the change in the result.
Worked example
A student investigates how water temperature affects how quickly sugar dissolves. The invented results are:
| Trial | Water temperature (°C) | Water volume (cm³) | Sugar mass (g) | Stirring | Time to dissolve (s) |
|---|---|---|---|---|---|
| A | 20 | 100 | 5.0 | gentle | 48 |
| B | 40 | 100 | 5.0 | gentle | 31 |
| C | 60 | 100 | 5.0 | vigorous | 12 |
Step 1, label the variables. Independent: water temperature. Dependent: time to dissolve.
Step 2, scan each column. Volume is 100 cm³ in every row and sugar mass is 5.0 g in every row, so both are controlled.
Step 3, find the slip. Stirring is gentle in A and B but vigorous in C. Stirring was not controlled in trial C.
Step 4, judge the effect. From A to B, only temperature changed, and the time fell by 48 − 31 = 17 s. From B to C the time fell by 31 − 12 = 19 s, but both temperature and stirring changed, so that drop cannot be blamed on temperature alone.
The mistake to watch for
Mistaken answer: “All the conditions were controlled because the same sugar and the same beaker were used.”
Listing items that were the same does not prove that everything relevant was the same. The student missed the column where the values differ. Check each factor row by row, and only then say it is controlled.
Check yourself
1. In the table above, what is the smallest change that would make trial C a fair comparison with B?
Show answer
Use gentle stirring in trial C, so only temperature differs between B and C.
2. A student compares plant growth in a bright and a shaded spot. The bright spot is also next to an open window. Name the controlled condition that is probably not controlled.
Show answer
Temperature (or air movement). The bright spot differs in more than light.
3. Why is “the same beaker” a weak controlled condition for the sugar investigation?
Show answer
A beaker is unlikely to affect how fast sugar dissolves, so naming it shows no understanding of what matters. Better choices are water volume, sugar mass and stirring.
Where this leads next
Next, describe a safe conceptual measurement improvement so the readings themselves carry less error. The scientific investigation critic lets you practise writing evidence-linked critiques, and the investigations practice set mixes everything.
Students often spot the slip when someone reads their answer back to them. A teacher in online one-to-one Combined Science tuition can do this with your own fair-test answers.