What you need to know
- Unit 3Decide whether repeat readings are appropriate, deal with health and safety, and explain how uncertainty or systematic error could be reduced.
Follow the school’s approved reagents and organisms. These notes do not authorise a protocol. Implications of biology are discussed only when a question asks for benefits, risks or context. They are not a substitute for the hazard control.
By the end, you should be able to…
- Explain when repeat readings improve a plan, and what they cannot remove.
- Pair a hazard with a control measure.
- Distinguish uncertainty in a reading from a systematic error.
Build your understanding
Repeats answer a particular problem
Repeat readings are appropriate when a single reading is likely to vary: end-points judged by eye, biological material that differs between samples, or a count such as bubbles. The repeats can be used to calculate a mean and to notice a result that disagrees with the others.
Repeats do not repair a systematic error. If every tube is left in a water bath that is actually 5 °C below the set temperature, repeating the tubes repeats the same bias. Say that explicitly if a question asks whether more repeats would solve the problem. Often they would not.
Name the hazard and the action
A useful risk line has two parts: the hazard, then what you will do about it. Illustrative pairs, not a local risk assessment, include eye protection when heating Benedict’s reagent, a rack and holder for hot tubes, no tasting of laboratory solutions, and a disinfectant and closed plates when growing microbes. Ethanol near a flame is a fire hazard: keep the flame and the ethanol apart in the way your teacher specifies.
“Be careful” and “wear a lab coat” without a named hazard do not show that you have judged this experiment. Do not invent concentrations, disposal routes or organism names the question did not give. If the question mentions a living organism, the control should match that organism and the procedure, including not culturing unknown microbes at human body temperature unless the question’s own method says so.
Uncertainty is not the same as bias
Uncertainty is the doubt in a reading, for example the limited resolutionThe smallest distance between two points that can still be seen as separate points. of a syringe or the judgement of a colour end-point. It can often be reduced by a finer instrument, a clearer end-point, or more carefully repeated readings whose spread you then report.
A systematic error shifts results in one direction: a balance that is not tared, a leaking respirometer, a timer started late every trial, or standards and unknowns heated for different times. Reducing it means changing the procedure, not just repeating it. The specification also allows a comment on benefits, risks and context when that is asked. Answer that part separately from the hazard controls of the practical itself.
| Action | Helps with | Does not fix |
|---|---|---|
| Repeat readings and a mean | Random scatter between trials | A bias present in every trial |
| A stated hazard control | That hazard | An unnamed or different hazard |
| A finer scale or clearer end-point | Uncertainty of each reading | A wrongly chosen variable |
| Calibrating or taring | A systematic offset of the instrument | Biological differences between organisms |
Make your reasoning visible
Answer “repeats?” with a reason
Say yes or no, then say which kind of error the repeats address. A yes without a reason is incomplete.
Keep implications in their box
If the question asks for a social or environmental context, do not answer with only goggles. If it asks for a hazard, do not answer with only a general benefit of the science.
Avoid this misconception
The claim: Doing the experiment three times removes systematic error.
The correction: Repeats can show scatter and support a mean. A systematic error is repeated too, unless the procedure or the instrument is changed.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
An illustrative colorimeter is used once at each of five sucrose concentrations. Why might repeat readings still be appropriate?
Show suggested answer
Each reading can vary because of pipette delivery, mixing or the instrument. Repeats would show that variation and allow a mean. They would not correct a blank that had been set wrongly for every tube.
Practice question
Retrieval and explanation · suggested answer
Write one hazard and one matching control for heating tubes in a water bath with Benedict’s reagent.
Show suggested answer
Hot liquid and the reagent are hazards. Wear eye protection, point tubes away from people, and use a holder rather than picking up hot tubes by the glass. Do not taste the contents. Local disposal rules apply to the reagent.
Practice question
Retrieval and explanation · suggested answer
A student starts the timer only after the mixture has already been reacting for a few seconds, in every trial. Are more repeats the right improvement?
Show suggested answer
No. The late start is a systematic error, so every trial is short by a similar amount. Start the timer at the moment the reaction is begun. Repeats would reproduce the same delay.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
An unfamiliar leaf-disc method sinks discs in a syringe and times how long they take to rise under a lamp. Suggest one repeat decision, one hazard control, and one possible systematic error. The discs and lamp are illustrative.
Show a hint
Separate scatter between discs from a bias in the whole set-up.
Explore the reasoning
Repeats are appropriate because discs and cutting differ; time several discs and use a mean. The lamp can be hot, so do not touch the bulb and keep solutions off electrical equipment. A systematic error would be a clock that is started only after some discs have already begun to rise, or a syringe volume that is misread in the same direction every time. The assumption is that light is the factor being tested and that temperature is separately controlled.
Recall this lesson
Lesson recall questions
Question 1 of 5: A candidate plans to investigate how temperature affects the initial rate of an amylase-catalysed starch hydrolysis. State the independent variable and the dependent variable.
Unit 3 planning
Practice question
Retrieval and explanation · suggested answer
A candidate plans to investigate how temperature affects the initial rate of an amylase-catalysed starch hydrolysis. State the independent variable and the dependent variable.
Lesson: P2: Apparatus and the measurements you will take
Try this from memory first.
Beyond the classroomA benefit is not a risk assessmentContext · health and environment · Enrichment, not required recall
One Health discussions connect human, animal and environmental wellbeing. That is a context for why some biological work matters. It does not replace the hazard and control you write for the apparatus in front of you. On the paper, answer the part that was asked.
Back to the spec: Unit 3 planning: health and safety controls are one skill. A comment on benefits, risks and context is a further skill, used when the question asks for it.
World Health Organization: One Health questions and answersQuestion
A plan says only that the investigation is beneficial because it studies disease. What has been left out of a risk answer?
Explore a possible response
The hazard of this procedure and the control measure. A possible benefit of the science is not eye protection, aseptic technique, or a safe way to heat a reagent.