OCR A Level Biology: Development of Practical Skills — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Development of Practical Skills revision notes

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1. Define independent variable, dependent variable and control variable. [3]

2. Explain the difference between accuracy, precision and reliability. [3]

3. A student investigates the effect of sucrose concentration on the mass change of potato discs.

(a) State the independent and dependent variables. [2] (b) Give three variables that must be controlled and explain why each matters. [6] (c) Explain why percentage change in mass is used rather than absolute change. [2] (d) Describe how the student should improve the reliability of the results. [2]

4. A balance reads to ±0.01 g and a disc has a mass of 2.50 g.

(a) Calculate the percentage uncertainty in a single reading. [2] (b) Explain how percentage uncertainty could be reduced. [2]

5. Explain the difference between a systematic and a random error, giving an example of each and how to deal with it. [4]

6. State whether a line graph, bar chart or scatter graph is appropriate for each, with a reason:

(a) mean height of plants grown in four different soil types [2] (b) rate of reaction against temperature [2]

7. Distinguish between a control variable and a control experiment. [3]

8. An enzyme investigation records these times to a fixed end point at 30°C: 38 s, 40 s, 39 s, 65 s, 41 s. Identify the anomaly, calculate the mean of the remaining readings, and state the rate. [4]

9. A student wants to test whether there is a significant difference between the mean leaf lengths of two plant species. Which statistical test should be used, and what does it mean if the calculated value exceeds the critical value at p = 0.05? [3]

10. State which statistical test is appropriate for testing an association between two continuous variables. [1]

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Answers

1. Independent — the variable deliberately changed by the investigator [1]. Dependent — the variable measured as a result [1]. Control — a variable kept constant so it does not affect the dependent variable [1].

2. Accuracy — how close a measurement is to the true value [1]. Precision — how closely repeated measurements agree with one another, regardless of whether they are correct [1]. Reliability — the extent to which repeats give consistent results, so the conclusion can be trusted [1].

3. (a) Independent: sucrose concentration [1]. Dependent: change in mass of the potato discs [1]. (b) Any three, each for 2 marks: temperature — affects the rate of osmosis and therefore the mass change in a fixed time [1] [1]. Time in solution — discs left longer would approach equilibrium more closely [1] [1]. Surface area / size of disc — a larger area allows faster water movement [1] [1]. Same potato / same tissue — different potatoes have different internal solute concentrations [1] [1]. (c) The discs will not all start with exactly the same mass [1], so percentage change allows a fair comparison between discs of different starting masses [1]. (d) Repeat each concentration at least three times and calculate a mean [1], and identify and discard anomalous results rather than including them in the mean [1].

4. (a) (0.01 ÷ 2.50) × 100 [1] = 0.4% [1]. (b) Use a larger mass, for example by weighing several discs together [1], since the absolute uncertainty stays the same while the measured value increases [1]. (Or use a balance with a finer resolution.)

5. Systematic — an error that shifts every reading by the same amount in the same direction, e.g. a balance that is not zeroed [1]; dealt with by calibrating or zeroing the instrument, since repeats will not remove it [1]. Random — an error that varies unpredictably in size and direction, e.g. judging when a colour change is complete [1]; reduced by taking repeat readings and calculating a mean [1].

6. (a) Bar chart [1], because soil type is a categoric (discrete) variable [1]. (b) Line graph [1], because both variables are continuous and there is an expected relationship between them [1].

7. A control variable is a factor kept constant during the experiment so it does not itself affect the dependent variable [1]. A control experiment is a separate, parallel run in which the factor under test is absent — for example, boiled enzyme in place of active enzyme [1] — used to confirm the observed effect is genuinely caused by that factor and not some other confound [1].

8. 65 s is anomalous and should be discarded [1]. Mean of the remaining readings = (38 + 40 + 39 + 41) ÷ 4 = 39.5 s [1] [1]. Rate = 1 ÷ 39.5 = 0.0253 s⁻¹ [1].

9. A t-test [1], since it compares the difference between two means [1]. If the calculated value exceeds the critical value, the difference is significant and the null hypothesis is rejected — meaning there is less than a 5% probability the difference arose by chance [1].

10. A correlation coefficient [1].

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Where marks are usually lost

• Confusing accuracy with precision.
• Naming control variables without saying why they matter.
• Forgetting that repeats improve reliability and the accuracy of the mean by reducing random error, but do nothing for a systematic error, which needs calibration.
• Using a bar chart for two continuous variables.
• Confusing a control variable with a control experiment.
• Calculating a mean before identifying and discarding anomalous readings.
• Choosing a correlation coefficient when comparing two means, or a t-test when testing an association.