How to read an experiment: variables, controls and valid conclusions
TEAS 7 outline: Use the scientific method to interpret an investigation
- Steps: observe, question, hypothesis, prediction, experiment, analysis, conclusion.
- The independent variable is what you change; the dependent variable is measured.
- Controlled variables stay the same; the control group skips the tested change.
- Larger samples, random assignment and repeated trials make results more reliable.
- Data can support or refute a hypothesis but never prove it.
Key facts
- The usual steps are: observe, ask a question, form a testable hypothesis, make an if-then prediction, experiment, analyze the results, and report; the process often loops back.
- The independent variable is changed or controlled on purpose by the researcher; the dependent variable is measured to see the effect.
- Controlled variables (constants) are conditions kept the same in every group so they cannot explain the results.
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- A control group is treated exactly like the experimental group except that it does not receive the tested change, so it serves as the baseline for comparison.
- Larger samples are more reliable than very small ones, and random assignment to groups makes it likely that the groups start out alike.
- Repeating trials and having other scientists replicate a study shows whether a result is consistent or just a fluke.
- A valid conclusion answers the original question, stays within the conditions tested, and says the data support or do not support the hypothesis.
Quick check1 question · 2 more at the end
A student wants to know whether water temperature affects how quickly a sugar cube dissolves. She fills four identical cups with 200 mL of water at 10 °C, 25 °C, 40 °C, and 55 °C. She drops one sugar cube from the same box into each cup, stirs each cup the same way, and times how many seconds it takes for the cube to dissolve completely.
What is the dependent variable in this investigation?
Correct answer: C.
The dependent variable is what the researcher measures to see the effect of the change she made on purpose. Here she changes the water temperature (the independent variable) and measures the dissolving time, so dissolving time is the dependent variable. A quick check is the sentence "Does _ affect _?": "Does water temperature affect dissolving time?" puts the independent variable first and the dependent variable second.
A Water temperature is what the student changes on purpose, so it is the independent variable, not the dependent one.
B The 200 mL volume is kept the same in every cup, so it is a controlled variable.
C Correct: dissolving time is what the student measures to see whether temperature has an effect.
D The 10 °C cup is one of the conditions being compared, not a variable; confusing a group or trial with a variable is a common mistake.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: The scientific method moves from observation and question to a testable hypothesis, an experiment, analysis, and a conclusion that others can check. In an experiment, the independent variable is what the researcher changes, the dependent variable is what is measured, and controlled variables stay the same. A control group, enough subjects, and repeated trials make conclusions trustworthy.
1 What are the steps of the scientific method?Real science isn't a straight line. 1 min
Here is the usual order, using a question you could test at home:
- Observe. Your basil plant on the kitchen counter looks pale and leggy.
- Ask a question. Does adding liquid plant food change how tall basil seedlings grow?
- Form a hypothesis. A testable, possible explanation: basil seedlings grow taller when they get plant food.
- Predict. Usually in if-then form: if plant food helps growth, then seedlings given plant food will be taller after 3 weeks than seedlings given none.
- Experiment. Change only the plant food, keep everything else the same, and measure.
- Analyze. Compare averages, look for a pattern.
- Conclude and report. Say whether the data support the hypothesis, then share the work so others can check it (peer review) and repeat it.
Real science isn't a straight line. A surprising result often sends you back to step 2 or 3 with a new hypothesis.
2 Which variable is independent, dependent or controlled?" The independent variable goes in the first blank and the dependent in the second. 1 min
This is where most points are won or lost, so use the basil experiment:
| Part | Meaning | In the basil experiment |
|---|---|---|
| Independent variable | What the researcher changes on purpose | Plant food per week (0, 5 or 10 mL) |
| Dependent variable | What is measured to see the effect | Seedling height after 3 weeks |
| Controlled variables (constants) | Conditions kept the same for everyone | Same seeds, soil, pot size, water, light, temperature |
| Control group | The baseline group that doesn't get the tested change | Seedlings given no plant food (0 mL) |
| Experimental groups | Groups that get the tested change | Seedlings given 5 mL or 10 mL per week |
A quick test: put the two variables into the sentence "Does ___ affect ___?" The independent variable goes in the first blank and the dependent in the second. "Does plant food affect height?" works; "Does height affect plant food?" makes no sense.
On a graph, the independent variable usually goes on the x-axis and the dependent variable on the y-axis.
3 What's the difference between a control group and controlled variables?They sound alike but are different things. 1 min
They sound alike but are different things.
- Controlled variables are conditions: water, soil, light, temperature. Every group gets the same amount.
- A control group is a group of subjects: plants, people, test tubes. It is set up exactly like the others but does not get the tested change, so you can see what happens without it.
In drug studies, the control group often gets a placebo, a look-alike with no active ingredient, because people's expectations alone can change how they feel. In a double-blind study, neither the participants nor the researchers working with them know who got what.
4 Why do sample size and repeated trials matter?One basil seedling might be tall just because it came from a stronger seed. 1 min
Living things vary. One basil seedling might be tall just because it came from a stronger seed. If each group had only one plant, that single plant could decide your result.
- Sample size: Use many subjects per group. Very small samples can be unreliable; larger samples are better.
- Random assignment: Let chance decide which subject goes in which group, so hidden differences (lurking variables) spread out evenly.
- Repeated trials: Run the test more than once and average the results. If repeats agree, the result is reliable.
- Replication: When other scientists repeat the study and get the same result, confidence grows.
5 How do you spot a flaw in an investigation?Read this one and look for problems before reading the list. 1 min
Read this one and look for problems before reading the list.
Ana wants to know whether playing music helps bean plants grow. She plays music all day to 2 plants on a sunny windowsill and keeps 2 plants without music in a shady hallway. After a month, the music plants are taller, and she concludes that music makes plants grow.
- Two variables changed at once. Music and light differ, so either could explain the difference. This is the biggest flaw.
- Tiny sample. Two plants per group is not enough.
- No repeats. One run, no averaging.
- Conclusion too strong. "Music makes plants grow" goes beyond what this test can show.
The fix: put all plants in the same spot, use more plants per group, assign them to groups at random, and repeat the test.
6 How do you write a valid conclusion? 1 min
A valid conclusion:
- Answers the original question, using the dependent variable's data.
- Stays inside what was tested. Results for 0–10 mL of plant food don't tell you what 50 mL would do.
- Uses "support," not "prove." Data can support or refute a hypothesis but never prove it.
- Notes limits, like a small sample or one type of plant.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: The control group is a group that does not get the treatment, not the same thing as controlled variables, which are the conditions held constant.
Watch out: The independent variable is the one the researcher changes, not the one that changes in response; the response is the dependent variable.
Watch out: Changing two things at once makes it impossible to tell which one caused the result; repeating that flawed test does not fix it.
Watch out: Repeated trials make results more reliable, but they do not make a result proven.
Watch out: Results apply to the conditions tested; a conclusion about untested amounts, organisms or settings goes beyond the data.
How to remember it
Memory hook: "DRY MIX." Dependent = Responding = Y-axis. Manipulated = Independent = X-axis.
Self-check:
- In "Does caffeine dose affect reaction time?", name the independent and dependent variables.
- Explain the difference between a control group and a controlled variable in one sentence each.
- Find the flaw: a student tests a new study app on 3 volunteers who already earn top grades.