Q: What are control variables and why are they important?

Control variables are factors that are kept constant throughout an experiment to ensure that only the independent variable affects the dependent variable. For example, in an enzyme experiment, you might control temperature, pH, and substrate concentration. They are important because if you fail to control a variable, you cannot be sure that any observed effect is due to the independent variable – the experiment would lack validity. In exam questions, you will often be asked to identify control variables and explain how to control them (e.g., using a water bath to maintain temperature).

Q: How do I write a risk assessment for a biology practical?

A risk assessment should identify hazards (e.g., chemicals, sharp instruments, microorganisms), evaluate the risk (likelihood and severity), and state control measures to reduce risk. For example, for using hydrochloric acid: hazard – corrosive; risk – skin burns; control – wear gloves and goggles, handle in a fume cupboard if concentrated. You should also include emergency procedures (e.g., wash affected area with water). In the Practical Endorsement, you are expected to carry out risk assessments before starting practical work. In written exams, you may be asked to suggest safety precautions for a given procedure.

Question 1

What is the difference between accuracy and precision in biology practicals?

Accepted Answer

Accuracy refers to how close a measurement is to the true value. For example, if the true pH of a solution is 7.0 and you measure 7.1, your measurement is accurate. Precision refers to how consistent repeated measurements are. If you measure pH three times and get 7.0, 7.0, and 7.1, your measurements are precise (low spread). You can be precise but not accurate (e.g., if your pH meter is miscalibrated, you might get 6.5 every time – precise but inaccurate). In exams, you need to suggest ways to improve both: use calibrated equipment for accuracy, and repeat measurements for precision.

Question 2

How do I calculate percentage uncertainty and why is it important?

Accepted Answer

Percentage uncertainty is calculated as (absolute uncertainty / measured value) × 100%. For example, if you measure 25 cm³ using a measuring cylinder with an uncertainty of ±0.5 cm³, the percentage uncertainty is (0.5/25)×100% = 2%. It is important because it tells you how reliable your measurement is relative to the size of the quantity. When you combine measurements (e.g., in rate calculations), you add percentage uncertainties to find the total uncertainty. In exams, you may be asked to calculate uncertainty or explain how it affects the reliability of your conclusion.

Question 3

What is the difference between reliability and validity in an experiment?

Accepted Answer

Reliability refers to the consistency of results – if you repeat the experiment under the same conditions and get similar results, it is reliable. This is improved by repeating measurements and calculating a mean. Validity refers to whether the experiment actually tests the hypothesis. For an experiment to be valid, you must control all variables except the independent variable, and your method must measure what you intend to measure. For example, if you are testing the effect of temperature on enzyme activity, you must keep pH and substrate concentration constant – otherwise the experiment is not valid. In evaluations, you should comment on both.

Question 4

How do I choose the right statistical test for my biology data?

Accepted Answer

The choice depends on your experimental design and data type. For comparing two means (e.g., control vs. treatment), use a Student's t-test if data is normally distributed and variances are equal. For comparing observed and expected frequencies (e.g., genetics ratios), use a chi-squared test. For correlation (e.g., relationship between two continuous variables), use Pearson's correlation coefficient (parametric) or Spearman's rank (non-parametric). In the A-Level course, you are expected to know when to use each test and how to interpret p-values (usually p<0.05 is significant). Always state the null hypothesis and whether you accept or reject it.

Question 5

What are control variables and why are they important?

Accepted Answer

Control variables are factors that are kept constant throughout an experiment to ensure that only the independent variable affects the dependent variable. For example, in an enzyme experiment, you might control temperature, pH, and substrate concentration. They are important because if you fail to control a variable, you cannot be sure that any observed effect is due to the independent variable – the experiment would lack validity. In exam questions, you will often be asked to identify control variables and explain how to control them (e.g., using a water bath to maintain temperature).

Question 6

How do I write a risk assessment for a biology practical?

Accepted Answer

A risk assessment should identify hazards (e.g., chemicals, sharp instruments, microorganisms), evaluate the risk (likelihood and severity), and state control measures to reduce risk. For example, for using hydrochloric acid: hazard – corrosive; risk – skin burns; control – wear gloves and goggles, handle in a fume cupboard if concentrated. You should also include emergency procedures (e.g., wash affected area with water). In the Practical Endorsement, you are expected to carry out risk assessments before starting practical work. In written exams, you may be asked to suggest safety precautions for a given procedure.

Module 1: Development of practical skills in biology

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

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Biodiversity

Biological membranes

Biological molecules

How to Revise Module 1: Development of practical skills in biology — OCR A-Level Biology

Examiner Tips for Module 1: Development of practical skills in biology

Common Mistakes in Module 1: Development of practical skills in biology

Key Marking Points