Planning & Evaluating an Investigation

Revise Planning & Evaluating an Investigation for Biology 3 (IAL) WBI13 (AS Level) — revision notes and instant AI marking.

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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

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