What you need to know
- Unit 6Plan an investigation to test a hypothesis: formulate a null hypothesis, and identify the independent, dependent and controlled variables so the test is fair.
Unit 6 (WBI16) is a written paper of 1 hour 20 minutes, out of 50 marks. Calculators may be used, and questions can draw on practical work from the whole course. These lessons are study guidance, not a mark scheme, and they do not replace the Core Practical guides. A null hypothesis is on this Unit 6 planning list. It was not on the Unit 3 list.
By the end, you should be able to…
- Write a null hypothesis as no difference or no association, using the variables in the question.
- Identify the independent, dependent and controlled variables that match that hypothesis.
- Keep a prediction separate from the null hypothesis.
Build your understanding
The null is the “no effect” statement
A Unit 6 plan can be asked to test a hypothesis. The null hypothesis is the statement you would keep if the factor made no difference: there is no difference between the means, or there is no association between the two variables. Write it with the measurements named.
An illustrative null is: “There is no difference in the mean number of plant speciesA group of organisms that can interbreed to produce fertile offspring. per 0.25 m² quadratA square frame of known area used to sample organisms. between the mown and the unmown area.” “Mowing reduces diversity” is a prediction, not the null. The prediction can sit beside the null if the question asks for both. Do not offer only the prediction when a null was asked for.
The variables have to match the sentence
The independent variable is the factor you change or the groups you compare. The dependent variable is the outcome you measure. Controlled, or standardised, variables are the factors you keep the same so they are not a second explanation.
In that quadrat example the independent variable is the management of the area, mown or unmown. The dependent variable is the number of species per quadrat. Quadrat area, the month, and the rule used to decide that a species is present are controlled variables. If the sentence of the null mentions a different outcome, rewrite the null or the variables until they describe one investigation.
A fair test is how the null could be wrong for one reason
State how each important control variableA variable kept the same throughout an investigation so that it does not affect the result. is held, not only its name. “Same quadrats” is weaker than “every quadrat is 0.5 m by 0.5 m, so the area is 0.25 m² in both areas.” Different areas would change the count even if the vegetation did not.
A control experiment is still a different idea from a controlled variable. A bare-soil comparison, or a dish without the treatment, is a control experiment. Name which one the question wants. The arithmetic of a later statistical test belongs with the data, and only if a test is asked for. Planning the null does not let you call a result significant.
NOT: the null is not your hoped-for result, and planning it does not let you call a result significant.
- Null means no difference or no association
- Name the measurement in that sentence
- The prediction is a separate claim
- Controlled variables are kept the same
Two traps this diagram blocks
- Do not write the prediction and call it the null.
- Do not claim the null has been proved if the means look similar.
From the diagram alone
Question
Write a null hypothesis for a comparison of mean gas volume from pondweed at two light intensities, and name the independent variable. [3]
Show a model answer
There is no difference in the mean volume of gas, in a stated time, between the two light intensities. The independent variable is light intensity.
| Sentence you write | It says | It does not say |
|---|---|---|
| Null hypothesis | No difference, or no association, between the named variables | The result you hope to find |
| Prediction | The direction or pattern you expect | A statement that can be left unchanged if nothing happens |
| Independent variable | The factor or groups being compared | The measurement you record |
| Controlled variable | A factor kept the same, with the method that keeps it the same | A control experiment that lacks the treatment |
Make your reasoning visible
Use the question’s two groups
If the question compares two woods, two temperatures or two extracts, those groups belong in the null. Do not swap in a core practical the question did not mention.
One difference only
Read the null back. If two factors change, a later difference cannot be blamed on one of them.
Key explanations
- The independent variable is the factor or the groups being compared.
- Controlled variables are kept the same, and you should say how, so only one factor can explain a difference.
Avoid this misconception
The claim: The null hypothesis is your scientific prediction written in formal language.
The correction: The null says there is no difference or no association. The prediction is the effect you expect. Unit 6 can ask for the null specifically.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
An illustrative investigation compares the mean volume of gas collected in 10 minutes from pondweed at two stated light intensities. Write a null hypothesis.
Show suggested answer
There is no difference in the mean volume of gas collected in 10 minutes between the two light intensities.
Practice question
Retrieval and explanation · suggested answer
For that investigation, name the independent variable, the dependent variable, and one controlled variable with how it is kept the same.
Show suggested answer
The independent variable is light intensity. The dependent variable is the volume of gas collected in 10 minutes. A controlled variable is temperature, kept at one stated value with a thermometer, or the same mass or length of pondweed in both conditions.
Practice question
Retrieval and explanation · suggested answer
A student writes “Light will increase photosynthesis, so the null hypothesis is that the lamp works.” What is wrong with that null?
Show suggested answer
It is not a statement of no difference in the measured outcome. A usable null would say there is no difference in the stated rate or volume between the light treatments.
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 plan records both soil moisture and the number of a named plant in the same quadrats, then says the null hypothesis is “moisture affects the plant.” Rewrite the null so it could be tested, and state which variable is not an independent variable you set yourself.
Show a hint
This is an association you observe, not a factor you dial to chosen values.
Explore the reasoning
A testable null is: there is no association between soil moisture and the number of that plant per quadrat. Soil moisture is measured, not set by the student, so it is not an independent variable in the sense of a factor you change on purpose. Both moisture and plant number are recorded in each quadrat. The assumption is that quadrat area and the moisture method stay the same.
Recall this lesson
Lesson recall questions
Question 1 of 5: A candidate plans to test whether light wavelength affects the rate of photosynthesis. State a suitable null hypothesis.
Unit 6 planning
Practice question
Retrieval and explanation · suggested answer
A candidate plans to test whether light wavelength affects the rate of photosynthesis. State a suitable null hypothesis.
Lesson: P1: A null hypothesis and the variables
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Failing to reject is not proof
P1 · A null hypothesis and the variablesThe NIST engineering statistics handbook describes a statistical test as a way to decide whether there is enough evidence to reject a null hypothesis. Not rejecting it can…Specification Unit 6 · PlanningRead & use the scienceClose article
Why this matters
The NIST engineering statistics handbook describes a statistical test as a way to decide whether there is enough evidence to reject a null hypothesis. Not rejecting it can mean the null is reasonable, or that the study did not collect enough evidence. That handbook is enrichment. It is not a Unit 6 mark scheme, and it does not supply a critical value for your question. The word ‘null’ can sound like a prediction that nothing will happen. It is instead the starting comparison that a test may challenge. A clinical or ecological study has to define the outcome and comparison groups before inspecting results. If investigators change the endpoint after seeing the data, the apparent evidence is harder to interpret. At A level, write the null in terms of the variables actually measured, not the result you hope for.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 6, pp. 39–42. The application extends beyond required recall.
- Unit 6 · Planning Plan an investigation: formulate a null hypothesis, identify variables, choose and calibrate apparatus, consider repeats, safety and ethics, and state how data will be used.
Use the science
A plan says that if the means are similar the null hypothesis has been proved. Why is that too strong?
Explore a possible response
Similar means may simply fail to provide enough evidence to reject the null. Failure to reject is not proof that the null is true, and a plan cannot know that outcome before the data exist.
Explore the source
A trial names its endpoint before it begins
P1 · A null hypothesis and the variablesAn NIH clinical-trials glossary defines an endpoint as the outcome selected for analysis of a treatment’s safety or benefit. That is a real-world version of the Unit 6…Specification Unit 6 · PlanningRead & use the scienceClose article
Why this matters
An NIH clinical-trials glossary defines an endpoint as the outcome selected for analysis of a treatment’s safety or benefit. That is a real-world version of the Unit 6 planning demand to identify the dependent variable and state a testable hypothesis. If a trial changes from a measured symptom score to a different outcome after seeing its results, its conclusion is harder to assess. School investigations are much smaller, but they should also name the groups, the measurement and the time point before collecting data. The null compares those named quantities.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 6, pp. 39–42. The application extends beyond required recall.
- Unit 6 · Planning Plan an investigation: formulate a null hypothesis, identify variables, choose and calibrate apparatus, consider repeats, safety and ethics, and state how data will be used.
Use the science
A plan says only “the treatment helps plants”. What would you add to make a null hypothesis testable?
Explore a possible response
Name the treatment and comparison groups, a measurable outcome such as mean shoot length, and a time point; then state that the mean outcome does not differ between groups.
Explore the source
- Endpoint: clinical trial glossaryNCATS, NIH