A fair plan changes one variable, measures one thing and keeps everything else the same. Write those three lists before you write any steps.
This lesson is part of plant nutrition. Wider planning ideas are in biological investigation.
What is the planning order?
- Aim: what do you want to find out?
- Variables: changed, measured and constant.
- Materials and method: short numbered steps.
- Control and repeats: what you compare against, and how many plants.
- Results and conclusion: how you will record and interpret.
Worked example: does nitrate affect growth?
An original aim: to find out whether nitrate affects the growth of young maize seedlings.
Variables. Changed: the nitrate in the culture solution (with or without). Measured: the height gain of each seedling after 3 weeks. Constant: the type and age of seedling, volume of solution, light, temperature and other minerals.
Method. Set up 2 groups with 5 seedlings each. Group A gets a complete solution, the control. Group B gets a solution with nitrate removed.
Measure the height of each seedling at the start and after 3 weeks. Calculate the average height gain for each group.
Results and conclusion. Suppose the averages are 12.0 cm for A and 4.5 cm for B. Conclusion: seedlings without nitrate grew less, because nitrate is needed to make proteins for new cells.
Why are five seedlings better than one?
A single seedling could be weak or strong by chance. Five in each group give five height gains, and their average is less affected by one unusual seedling.
The repeats also show the spread. If the five values in a group are close, the result is more reliable.
The mistake that makes the test unfair
A student puts group B in a darker place “to save space”. Now two things differ: the nitrate and the light. A smaller height gain could come from either.
Keep light, temperature and volume identical for both groups, and change only the nitrate.
A self-check question
Plan a test on whether phosphate affects root growth. State the changed variable, the measured variable and two constant variables.
Answer
Changed: phosphate in the solution (present or absent). Measured: root length after a set time. Constant, any two of: type and age of seedling, volume of solution, light, temperature and the other minerals. Include a control with all minerals and use at least five seedlings in each group.
Record any weak points in the mistake log tool. Then try the practice set.
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