LOGIC LAB · UNITS 3 AND 6
Unit 3 (WBI13) and Unit 6 (WBI16) both ask you to plan an experiment. Build the plan on CORMMSS: seven points, one sentence each. Then add the A level extras that turn a method into a plan you can defend.
Planning is based on the practical work in Units 1 and 2. Expect parts that ask you to name the variables, say how to control them, describe how to measure, deal with safety, and say how you would use the data and reduce uncertainty.
Planning draws on practical work from Units 1, 2, 4 and 5, and often starts from a hypothesis. On top of the Unit 3 skills, expect a null hypothesis, calibration, ethics for living organisms and a statistical test. A full plan can be a long answer, so structure matters.
These are Scholar Lab study notes, written to help you plan. They are not Pearson’s wording or an official mark scheme.
Write the seven points in this order and you will not miss a basic mark. At A level, each one needs a number, a unit or a method, not just a name.
Name the independent variable, how each value is produced and the values themselves: usually five or more, at even intervals, for example 20 to 60 °C in 10 °C steps from a thermostatically controlled water bath, or a serial dilution. Add a control when it shows the effect comes from the treatment, such as boiled enzyme or no inhibitor.
For organisms, name the species and match them on age, size, sex, developmental stage or source. For tissue or extracts, take them from the same organism or batch. Say how you would choose them, for example at random from one population.
Give the number of repeats, for example at least three at each value, and calculate a mean. At this level, say why: repeats show variation, let you spot anomalies and allow a standard deviation or a statistical test.
Name the dependent variable, the instrument and the unit, and say how the reading becomes a rate or a result: time to a colour end-point compared with a standard, volume of gas in a gas syringe, mass change as a percentage, or 1/time.
State the time interval or end-point: readings every 30 s for 5 minutes, or after a stated number of days. Include an equilibration time before you start, so every tube begins at the planned value.
Name the most important variable that could also affect the result, give its value and how it is controlled and checked: “pH 7, kept with a buffer solution”.
Name a second, different controlled variable in the same way: “the same volume and concentration of substrate, measured with a graduated pipette”. Monitoring counts too: check the water bath with a thermometer during the run.
These sit on top of CORMMSS. Unit 3 plans often need risk, uncertainty and data use. Unit 6 plans add the null hypothesis, ethics and the statistical test.
Unit 6 plans often start with a testable hypothesis, and a null hypothesis for the statistical test. The null hypothesis says there is no difference or no correlation.
Write both in terms of the actual variables, not “the factor affects the result”.
For example: Null hypothesis: there is no correlation between the distance from the woodland edge and the mean leaf area of the plants.
Choose a range wide enough to show the pattern, and include values on both sides of any optimum. Five or more values at even intervals make a line graph worth drawing.
Justify the range with biology: enzymes denature above about 60 °C, and most organisms die well before that.
For example: 20, 30, 40, 50 and 60 °C, so that the rise, the optimum and the fall are all visible.
A pilot study is a quick trial before the real run. It checks that the method gives readings you can measure and lets you fix the range, the intervals, the timing or the concentrations.
Say what the pilot will decide, not just “do a pilot study”.
For example: Run a pilot at 20 °C and 60 °C to check that the colour change takes between 30 s and 10 min, then adjust the enzyme concentration if it does not.
Name a specific hazard, the harm it could cause and a precaution that reduces the risk. Generic safety rules (“wear a lab coat”) earn little on their own.
Match the precaution to the hazard: eye protection for an irritant splashing, aseptic technique and disinfectant for microorganisms, heat-proof gloves for a hot water bath.
For example: Trypsin is an irritant if it gets on the skin or in the eyes, so wear eye protection and gloves, and wash off any splashes at once.
If the plan uses living organisms, say how you will look after them: use the fewest you need, avoid harm and stress, return animals to their habitat, and keep the time they are out of it short.
If people are involved, ask for informed consent and let them stop at any time. Do not use intensities that could harm them.
For example: Keep the water fleas at each temperature for only 5 minutes, use the smallest number that still allows repeats, and then return them to the culture.
Choose the test from the type of data and the question you are asking: Student’s t-test for a difference between two means, a correlation coefficient (such as Spearman’s rank) for a relationship between two variables, and chi-squared for observed and expected frequencies.
Say how you will use it: compare the calculated value with the critical value at p = 0.05, then accept or reject the null hypothesis. Plan enough repeats or pairs for the test to work.
For example: Collect at least 10 pairs of readings, so a Spearman’s rank correlation coefficient can be tested at p = 0.05.
Validity: the method measures what it sets out to measure, and only the independent variable is changing the result. Controlled variables protect validity.
Reliability: repeated readings agree. Repeats, means and standard deviations show this. Accuracy: readings are close to the true value. Calibration (checking a meter reads zero, or using a colour standard) protects accuracy. Precision is how closely repeats cluster, and it is limited by the resolution of the instrument.
For example: A colorimeter is set to zero with a blank before each reading, so the results are more accurate. Taking three repeats and a standard deviation shows how reliable they are.
Name a source of uncertainty or systematic error in your own plan, say how it affects the results and how to reduce it. Then say what you will do with the data: the table, the graph and the test.
An end-point judged by eye, temperature drift and natural variation between organisms are common limitations. Each needs its own fix.
For example: Judging the end-point by eye is subjective, so compare each tube with a colour standard or use a colorimeter.
This order follows how you would run the experiment, which keeps a 10-line answer readable. CORMMSS is still the checklist.
Worked plan · illustrative values
The question
Plan an investigation into the effect of temperature on the rate at which trypsin breaks down the protein in milk. The milk goes from cloudy to clear as the protein is digested.
The CORMMSS rows carry the method. The extras add what Unit 3 also asks for: safety, a way to reduce uncertainty, and how the data will be used. All values are illustrative.
Worked plan · illustrative values
The question
A student thinks ivy leaves are larger in shade. Plan an investigation to test whether there is a correlation between light intensity and the surface area of ivy leaves at the edge of a wood.
CORMMSS still works when you cannot set the variable: “Change” becomes the gradient you sample along, and “Same” includes the factors you record but cannot fix. The Unit 6 extras, a null hypothesis, a statistical test and ethics, sit on top. All values are illustrative.
Three original prompts. Plan each one on paper, then check it against the points. The points are Scholar Lab guidance, not an official mark scheme.
Practice 1
Your turn
Plan an investigation into the effect of sucrose concentration on the rate of respiration of yeast. Use a redox indicator, methylene blue, which loses its blue colour as it is reduced.
Write your plan on paper first, one sentence per letter. Then open the hints or the points.
Practice 2
Your turn
Plan an investigation to test the hypothesis that temperature affects the heart rate of water fleas (Daphnia). Include a null hypothesis, how you would analyse the data, and any ethical issues.
Write your plan on paper first, one sentence per letter. Then open the hints or the points.
Practice 3
Your turn
Plan an investigation to find out whether the mean height of limpet shells differs between an exposed and a sheltered rocky shore.
Write your plan on paper first, one sentence per letter. Then open the hints or the points.
