Scientific Method and Experimental Variables
How to design a fair test, identify variables and evaluate an experiment.
The scientific method
Scientists generally work in a cycle: observe, ask a question, form a hypothesis, test it with an experiment, analyse results and draw a conclusion. Conclusions can lead to new questions. The method is a flexible approach, not a rigid checklist.
Types of variable
- Independent variable: the one you deliberately change.
- Dependent variable: the one you measure.
- Control variables: the ones you keep the same so the test is fair.
Example. To investigate how light intensity affects the rate of photosynthesis in pondweed:
- Independent: light intensity (for example, distance of a lamp).
- Dependent: rate of photosynthesis, measured by counting bubbles per minute.
- Control: temperature, type and size of plant, carbon dioxide concentration.
A fair test
A fair test changes only one variable at a time. Otherwise you cannot tell which change caused the result.
Reliable, accurate and valid
- Reliable: repeating gives similar results.
- Accurate: results are close to the true value.
- Valid: the experiment measures what it claims to measure.
Handling data
- Repeat measurements and calculate a mean.
- Ignore or investigate anomalous results, and explain why.
- Plot the independent variable on the x-axis and the dependent variable on the y-axis.
Evaluating an experiment
Comment on:
- Sources of error and how to reduce them.
- Whether enough repeats were done.
- Whether control variables were really controlled.
- Whether the conclusion is supported by the data.
Common mistakes
- Changing more than one variable.
- Confusing independent and dependent variables.
- Drawing conclusions the data does not support.
Quick recap
Change one thing, measure one thing, control the rest, repeat, and conclude only what the evidence shows.