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AS Level · Biology 3 (IAL) WBI13

Reliability, Validity & Suggesting Improvements

Revise Reliability, Validity & Suggesting Improvements for Biology 3 (IAL) WBI13 (AS Level) — revision notes, 2 practice questions and instant AI marking.

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Edexcel IAL Biology • Unit 3: Practical Skills in Biology I

Reliability, Validity & Suggesting Improvements

🔍 Big idea: Reliability and validity are not the same thing, and the improvement that fixes one will not fix the other. A reliable experiment gives the same answer every time; a valid one measures what it claims to. You can be perfectly reliable and completely invalid.

Summary — What This Topic Covers

What reliable means, and how repeats improve it
What valid means, and why controlled variables decide it
Why an experiment can be reliable but not valid
Writing improvements that are specific enough to score
The difference between an improvement and an extension

1. Reliability

Key Term

A reliable result is one that would be obtained again if the experiment were repeated under the same conditions. It is about the consistency of the measurement, not its truth.

Judged by how closely the repeats agree with each other
Improved by taking more repeats and calculating a mean
Improved by reducing random error — finer apparatus, more consistent technique
Shown on a graph by short error bars and little scatter about the line

How to describe it

"The three repeats at 30 °C ranged from 13.5 to 14.5 cm³, a spread of only 1 cm³, suggesting the results are reliable." Quote the spread — that is the evidence.

2. Validity

Key Term

A valid experiment measures what it set out to measure. It requires that only the independent variable changed, and that the dependent variable genuinely reflects the thing being investigated.

Validity is decided by the controlled variables. If pH drifted while you varied temperature, any change in rate could have come from either, and the experiment cannot answer the question it asked — however consistent the numbers were.

Every relevant variable other than the IV must be held constant
A control is needed to show the effect is caused by the IV and not by the procedure itself
The measurement must actually reflect the biology — counting bubbles is a poor proxy for oxygen volume, because bubble size varies

The one they test

An experiment can be reliable but not valid: three repeats agreeing perfectly, all measuring the wrong thing. It can never be valid but unreliable in any useful sense — if you cannot reproduce it, you cannot claim it measured anything.

3. Controls

Key Term

A control is a run in which the independent variable is absent or inactive, everything else being identical. It shows what would have happened anyway.

INVESTIGATION CONTROL ───────────────────────────────────────────────────────────────── effect of enzyme on starch boiled (denatured) enzyme effect of light on rate tube wrapped in foil effect of a solution on mass distilled water, 0 mol dm⁻³ effect of an antibiotic disc disc soaked in sterile water

Note that a good control is not simply "leave it out" — it is the same procedure with the active factor removed. Boiled enzyme controls for the volume, the protein and the liquid; omitting the enzyme entirely does not.

4. Improvements That Score

The structure

Name the problem, then the specific change, then what it fixes. Three parts, as with risk assessment.

5. Improvement vs Extension

The difference

improvement extension

Practice Questions

Practice Question 1

Practice Question 2

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