Revise Planning & Evaluating an Investigation for Biology 3 (IAL) WBI13 (AS Level) — revision notes and instant AI marking. Free to start.
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Edexcel IAL Biology • Unit 3: Practical Skills in Biology I
Planning & Evaluating an Investigation
🗺️ Big idea: Planning and evaluating are the same skill viewed from opposite ends. Everything you should have controlled is what the evaluation will ask about, and every weakness you admit at the end was a decision you made at the start. Learn the shape once and both halves of the paper open up.
Summary — What This Topic Covers
The full structure of a plan: variables, method, controls, safety, analysis
The three kinds of variable and how to name them precisely
What an evaluation must contain to reach the top band
Linking each weakness to its effect on the result
How this topic frames the ones that follow
1. The Shape of a Plan
1 HYPOTHESIS direction + range + biological reason
2 VARIABLES independent (with range and levels)
dependent (how measured, in what units)
controlled (each with WHY it matters)
3 METHOD numbered, specific, repeatable by someone else
apparatus named with resolution
4 CONTROL the run with the factor removed
5 REPEATS how many, and why
6 SAFETY hazard → harm → precaution
7 ANALYSIS what you will calculate and plot
Easy marks
Marks are allocated section by section. Even a weak method still earns the variables, control, repeats and safety marks — provided you wrote them down. Never skip a heading because you are unsure of the biology.
2. The Three Variables
Key Terms
Independent variable — the one you deliberately change. Dependent variable — the one you measure. Controlled variables — everything else that could affect the dependent variable, held constant.
✗ "Control the temperature."
✓ "Keep the temperature at 30 °C using a thermostatically controlled
water bath, because enzyme activity increases with temperature and
would otherwise change the rate independently of pH."
A controlled variable needs THREE things:
the variable · the value + how it is held · why it matters
Common mistake
Listing controlled variables without saying why each one matters. The list is worth little; the reasons are where the marks are.
3. What an Evaluation Must Contain
Reliability — how closely did the repeats agree? Quote the spread
Anomalies — were there any, were they excluded, what might have caused them
Errors — name them and classify each as random or systematic
Validity — was anything else changing that should not have been
Improvements — specific changes, each matched to the fault it fixes
Confidence — does the conclusion actually follow from the data
The linking sentence
Every weakness should say what it did to the result. "The water bath fluctuated by ±3 °C, so the rate at each nominal temperature varied during the run, adding random scatter to the points."
What mark schemes look for
The effect on the result is the mark. Naming a problem without saying what it did to the data is half an answer.
4. Judging the Conclusion
The last thing an evaluation should do is ask whether the data supports the conclusion at all.
Is the difference between conditions larger than the spread within them? If the repeats overlap, the difference may be nothing
Was the range wide enough to see the whole pattern
Does the conclusion claim causation where only correlation was shown
Is the conclusion limited to the conditions tested — one species, one temperature range, one time of day
The overclaim
"This proves enzymes work best at 40 °C" — from one enzyme, one pH and three repeats. Say "these results suggest that, for this enzyme under these conditions…". Hedging correctly is a mark, not weakness.
5. Where the Rest of This Unit Fits
PLANNING Writing a Hypothesis & Choosing a Method
Making Solutions & Serial Dilutions
Safety, Risk Assessment & Ethics
MEASURING Apparatus, Measurement & Uncertainty
Resolution, Precision & Significant Figures
Biological Techniques & Slide Preparation
RECORDING & ANALYSIS Recording Data: Tables, Units & Repeats
Graphs: Choosing, Drawing & Reading Them
EVALUATING Anomalies, Random & Systematic Error
Reliability, Validity & Suggesting Improvements
Practice Questions
Practice Question 1
A student writes: "Controlled variables: temperature, pH, volume." Explain why this would not gain full marks and rewrite one of them.
Practice Question 2
Explain why an evaluation that says "there may have been errors in the experiment" gains no credit.
Practice Question 3
A student concludes "amylase works best at 40 °C" after testing 20, 30 and 40 °C. Comment on this conclusion.
Practice Question 4
Two conditions give mean values of 12.4 and 13.1 cm³, with repeats ranging from 10 to 15 cm³ in both cases. Comment on whether the difference is meaningful.
What to Memorise
Plan: hypothesis, variables, method, control, repeats, safety, analysisIV = changed, DV = measured, CV = held constantEvery CV needs value + how + whyEvaluation: reliability, errors, validity, improvementsSay what each weakness did to the resultQuote the spread as evidenceNever claim beyond the range testedOverlapping ranges → no real difference
Concepts Checklist
Exam Tips
What mark schemes look for
Reasons, not lists. A controlled variable earns its mark from the "because", and a weakness earns its mark from the effect it had.
The trap
Concluding that the highest value tested is the optimum. If your range stopped there, you have not shown it — you have run out of data.
Easy marks
Write every heading even if a section is thin. Variables, control, repeats and safety marks are available independently of how good the method is.
Worth remembering
If the repeats within each condition overlap, the difference between the means is probably not real. Saying so is a sign of judgement, not of a failed experiment.