TL;DR:
Every experiment has one independent variable that you change, one dependent variable that you measure, and controlled variables that you keep the same.
A control group shows what happens without the change, so the comparison means something.
A good prediction names the direction of the change and gives a biological reason, then the data either supports or does not support it.
Experimental design questions ask you to think like the person running the experiment. Someone describes a study, and you have to say what was changed, what was measured, what was kept the same and why a comparison is fair. The biology is often simple. The difficulty is keeping the three kinds of variable straight, and writing a justification that uses the evidence in the setup.
Three kinds of variable and a control
The independent variable is the factor the experimenter deliberately changes. The dependent variable is the factor measured in response. Controlled variables are every other factor kept the same across the groups, so that only the independent variable can explain any difference in the results.
A control group is the group that does not receive the changed condition, or receives the standard condition. It is the reference point. Without it, you can't tell whether a change in the results came from your treatment or from something that would have happened anyway.
Worked example: identifying variables in a plant growth experiment
Here is a setup written for this article. A student wants to test whether the amount of light a bean plant gets changes how tall it grows. Fifteen bean seedlings of the same species, all the same starting height, are split into three groups of five. Each group gets the same soil, pot size and watering, and is kept at the same room temperature. The groups get 4, 8 or 12 hours of light per day for 14 days. At the end, each plant's height is measured in centimeters.
Independent variable: the hours of light per day, which the student sets deliberately. It has three levels: 4, 8 and 12 hours.
Dependent variable: the height of each plant after 14 days, in centimeters. This is what the student measures.
Controlled variables: the species, starting height, soil, pot size, amount of water, temperature and number of days. These are kept the same so they can't explain the differences.
Control group: the 8-hour group is the reference, because it receives the standard light condition that the student uses for other plants. Each change is compared with it.
Notice that the number of plants per group matters too. Five plants per group gives a more reliable average than one plant per group, because a single unusual seedling would otherwise decide the result.
Compare the groups with the mean
Suppose the final heights, in centimeters, are as follows. These numbers are hypothetical, written for this article.
Plant | 4 h light (cm) | 8 h light (cm) | 12 h light (cm) |
|---|---|---|---|
1 | 10 | 15 | 18 |
2 | 12 | 16 | 19 |
3 | 11 | 14 | 17 |
4 | 9 | 15 | 20 |
5 | 13 | 17 | 18 |
Total | 55 | 77 | 92 |
Mean (total ÷ 5) | 11.0 | 15.4 | 18.4 |
Check the totals first: 10 + 12 + 11 + 9 + 13 = 55, 15 + 16 + 14 + 15 + 17 = 77, and 18 + 19 + 17 + 20 + 18 = 92. Then the means: 55 ÷ 5 = 11.0 cm, 77 ÷ 5 = 15.4 cm, and 92 ÷ 5 = 18.4 cm.
Compare each group with the control. The 12-hour group is 18.4 − 15.4 = 3.0 cm taller on average than the 8-hour control. The 4-hour group is 15.4 − 11.0 = 4.4 cm shorter than the control. Those are the differences a grader would expect you to name, with the numbers that produce them.
Write a prediction you can justify
A prediction should state the direction of the change and give a biological reason. Here the reason is that light drives photosynthesis, which makes the sugars the plant uses to grow. More light, within the range the plant can use, should mean more growth. That is the reasoning, and the data either supports it or does not.
A good justification also states the limit. Light is only one factor, and above some level other things such as water, temperature or nutrients may limit growth. Mentioning the limit shows you understand that the relationship is not unlimited.
Tip: When a question asks you to justify a prediction, write it as: prediction, biological reason, then the evidence from the data that supports or contradicts it.
Mistakes to avoid
Calling the factor you measured the independent variable. The independent variable is the one you change.
Forgetting a controlled variable. If the pots were different sizes, the size could explain the results.
Using a single plant per group, which can be misleading.
Writing a prediction with no reason, or a reason with no link to the data.
Your plan for this week
Take three experimental setups from released AP Biology questions, and label each variable and the control in the margin.
Write one prediction for each, with its biological reason, in two sentences.
Check your reasoning against the official scoring guidelines for the current year.
Redo a setup you got wrong, and write what the control group was for in one line.
Design questions reward the same habit every time: name the change, the measurement, the kept-same factors and the reference group. Once you do that consistently, the rest of the question becomes a matter of reading the data carefully.
Frequently asked questions
Can a study have more than one independent variable?
Some do, but a standard question usually changes one factor at a time, so the comparison stays clear. Check the setup carefully and name each one if there is more than one.
Is a control group always a group that gets nothing?
No. It may receive the standard condition, as in the plant example, or a placebo-like condition with no change. Name which one it is in your answer.
What if the data does not match my prediction?
Say so, then explain what the results show and what else might account for them. A clear explanation of a mismatch is a strong answer.
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