OCR A Level Biology A: Planning, implementing, analysing and evaluating practicals (H420) – Revision Notes

Condensed recall for section 1.1 of OCR A Level Biology A (H420, Version 4.1, April 2026, first assessment in 2025), outcomes 1.1.1 to 1.1.4. For full explanations and worked examples, use the study guide. Test yourself with the practice set, the free diagnostics, the OCR A Level Biology hub and the printable checklist.

1.1.1 Planning

Part	Recall
(a) Design	Independent variable with a range of values; dependent variable with how and in what unit it is measured; apparatus and technique chosen; repeats; biology from the specification applied
(b) Control	Name each variable that could affect the dependent variable and say how it is held constant
(c) Appropriate?	Does the method measure what the aim needs, at a suitable scale, with enough values and repeats?

Plan in steps

1. State the aim and what you will change and measure.
2. Choose apparatus that suits the size of the quantity.
3. List control variables with a method for each.
4. Describe the method in order, with volumes, times and temperatures.
5. State repeats and how you will process the results.

Control variable versus control experiment. A control variable is kept the same. A control experiment is a parallel set-up lacking the factor under test, such as boiled enzyme or water in place of the solution.

1.1.2 Implementing

• Use apparatus correctly: meniscus read at eye level, balance zeroed with the container, graticule calibrated against a stage micrometer, colorimeter zeroed with a blank, dilutions made from measured volumes.
• Units belong in table headings and axis labels.
• Record raw data to the resolution of the instrument.
• Describe qualitative observations exactly (colour, precipitate, what changed).

Conversion	Value
1 cm³	1000 mm³
1 mm	1000 µm
Rate unit	quantity per unit time, such as cm³ s⁻¹
Magnification	image size ÷ actual size (same units)

Dilution reminder: 10 cm³ of 1.0 mol dm⁻³ stock made up to 40 cm³ gives 0.25 mol dm⁻³.

1.1.3 Analysis

Formulas

Quantity	Calculation
Mean	sum of values ÷ number of values (exclude anomalies, say so)
Range	highest − lowest
Percentage change	(final − initial) ÷ initial × 100
Rate from time	1 ÷ time to a fixed end point
Gradient	(y₂ − y₁) ÷ (x₂ − x₁), unit = y unit ÷ x unit
Rate at a point on a curve	gradient of the tangent at that point
Initial rate	gradient of the tangent at time zero

Significant figures

The answer cannot be more precise than the least precise value that went into it. Round at the end only.

Graph checklist

1. Independent variable on the x-axis, dependent on the y-axis.
2. Each axis labelled with quantity and unit.
3. Scale fills more than half the grid, equal steps.
4. Points plotted accurately.
5. Smooth curve or straight line of best fit through the trend; points not joined.
6. Chart type matches data.

Data	Display
Categories	Bar chart
Continuous data in classes	Histogram
Two variables, looking for a relationship	Scattergram, line of best fit
Continuous independent variable	Line graph

Gradients and intercepts

• Read two widely spaced points on the line, not clustered ones.
• The y-intercept is the value of y when x is zero, such as respiration alone in a light-intensity experiment.
• The x-intercept is where y is zero, such as the compensation point.
• For a tangent, the ruler touches the curve at one point and mirrors it on both sides.

Valid conclusions

Match the aim, quote data, claim no more than the range tested supports, explain with biology. Correlation alone does not show cause.

1.1.4 Evaluation

Term	Meaning	Check
Anomaly	A result that does not fit the repeats or trend	Compare with repeats or the line of best fit; exclude with a reason, or repeat
Precision	Closeness of repeat readings to each other	Small range
Accuracy	Closeness to the true value	Compare with an accepted value
Uncertainty	Margin of error in a reading, commonly half the smallest division	Percentage uncertainty = uncertainty ÷ value × 100
Percentage error	(measured − accepted) ÷ accepted × 100	Size of the difference from the accepted value

Combining uncertainties

1. Difference of two readings: add the absolute uncertainties.
2. Quotient such as a rate: add the percentage uncertainties.
3. Report the result with its uncertainty.

Small differences give large percentage uncertainties. A length of 18 mm read with a ruler to ±0.5 mm at each end has ±1.0 mm in total, which is 5.6 %.

Limitation to improvement

Limitation	Improvement
Coarse scale on apparatus	Finer divisions, or a larger quantity measured
Temperature drifts	Thermostatic water bath, thermometer check
Few values of the independent variable	More values across a wider range
Colour judged by eye	Colorimeter
Natural variation	Larger sample, calculate the mean

Worked reminders

Anomaly and rate. Times to a fixed end point: 22, 24, 23, 35, 23 s. The 35 s value is far from the others, so exclude it and say so. Mean = (22 + 24 + 23 + 23) ÷ 4 = 23.0 s. Rate = 1 ÷ 23.0 = 0.0435 s⁻¹ (3 significant figures).

Uncertainty in a rate. A syringe marked every 0.1 cm³ is read at the start and end, giving 4.0 cm³ in 50 s, with the stopwatch read to ±0.5 s.

each volume reading +/-0.05, so the difference is +/-0.10 cm3
volume: 0.10 / 4.0 x 100 = 2.5 %
time:   0.5 / 50 x 100 = 1.0 %
rate = 4.0 / 50 = 0.080 cm3 s^-1, uncertainty 2.5 + 1.0 = 3.5 %
rate = 0.080 +/- 0.003 cm3 s^-1

Must-know distinctions

• Independent variable (you change it) versus dependent variable (you measure it).
• Control variable (kept constant) versus control experiment (factor under test absent).
• Precision (repeats agree) versus accuracy (close to the true value).
• Uncertainty (the margin on a reading) versus percentage error (difference from an accepted value).
• Gradient of a straight line versus gradient of a tangent (rate at one point on a curve).
• Anomaly (does not fit) versus limitation (a weakness in the method).

Quick self-test

1. 31.2 cm³ of gas is collected in 25 s. Give the rate with the correct number of significant figures.
2. A measuring cylinder is marked every 1 cm³. What is the percentage uncertainty in a single reading of 20 cm³?
3. A line of best fit passes through (2, 5) and (10, 21). What is the gradient?
4. Mass falls from 3.20 g to 2.88 g. Calculate the percentage change.
5. A student measures 9.6 cm³ where the accepted value is 10.0 cm³. Calculate the percentage error.
6. Convert 0.35 mm to µm.
7. A cell is 0.12 mm long and its image is 36 mm. What is the magnification?
8. A balance reads ±0.005 g each time. A mass change of 0.40 g is found from two readings. What is the percentage uncertainty in the change?
9. A rate is calculated from a volume (5.0 % uncertainty) and a time (2.0 % uncertainty). What is the percentage uncertainty in the rate?
10. Distinguish precision from accuracy.
11. Why is the gradient of a tangent at time zero used for enzyme activity?
12. Give two features of a well-labelled graph axis.

Answers

1. 31.2 ÷ 25 = 1.248, so 1.2 cm³ s⁻¹ (2 significant figures, from the time).
2. ±0.5 ÷ 20 × 100 = 2.5 %.
3. (21 − 5) ÷ (10 − 2) = 2.0 (y unit per x unit).
4. (2.88 − 3.20) ÷ 3.20 × 100 = −10.0 % (a decrease of 10.0 %).
5. (10.0 − 9.6) ÷ 10.0 × 100 = 4.0 %.
6. 0.35 × 1000 = 350 µm.
7. 36 ÷ 0.12 = ×300 (convert 0.12 mm and 36 mm into the same unit first, as both are in mm).
8. Uncertainty in the change is ±0.01 g; 0.01 ÷ 0.40 × 100 = 2.5 %.
9. 5.0 + 2.0 = 7.0 %.
10. Precision is how close repeat readings are to each other; accuracy is how close a reading is to the true value.
11. The substrate concentration is highest and has changed least, so the rate is not yet slowed by substrate being used up.
12. The quantity and its unit, and a scale in equal steps that uses most of the grid.

Where marks are usually lost

• Naming a variable to control but not saying how.
• Confusing a control variable with a control experiment.
• Giving a calculated answer with more significant figures than the data.
• Leaving units out of table headings, axes or final answers.
• Reading a gradient from two points that are close together, or using data points that are not on the line.
• Joining plotted points with straight lines.
• Taking the uncertainty of a difference to be the uncertainty of one reading.
• Calling a tight set of repeats “accurate”.
• Writing “human error” instead of naming the cause.
• Suggesting “repeat more times” without saying what that improves.

Official syllabus

OCR Level 3 Advanced GCE in Biology A (H420), Version 4.1, April 2026, for first assessment in 2025, published by OCR. Section 1.1 and the mathematical requirements in section 5d.