Measure body length and body mass while preserving units and repeat observations
Define the feature and protocol before comparing a continuous sample.
Section B17.1 · Variation
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall genes, alleles, genotype and phenotype from the inheritance pathway. Use the stated organism, units and sample; the records in these lessons are invented.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must a measurement protocol supply?
What you will learn
- Plan comparable body measurements
- Interpret repeat readings and their limits
Original teaching model. Measurements, categories, DNA fragments and comparison records are invented; assumptions and sample boundaries are stated.
| Feature | Meaning |
|---|---|
| Length | Use stated endpoints and mm |
| Mass | Use a suitable tared balance and g |
| Repeat readings | 51, 52, 51 mm: mean ≈ 51.3 mm |
| Independent specimens | Three readings can still be one pod |
Plan comparable body measurements
An investigation of continuous variation starts with a defined feature, not a vague impression of size. For pod body length, choose endpoints and an orientation that can be applied consistently. Record length in millimetres with a ruler whose scale is known. For body mass, use a suitable balance, zero or tare it as appropriate and record grams. Measure comparable same-species specimens at a specified stage; length and mass are different variables.
Use a sampling method that does not deliberately pick only large or only small individuals. Keep the same measurement procedure across the sample and record all valid observations. If a pod is curved, state whether the protocol follows its curve or measures a straight distance between endpoints; mixing these methods introduces an artificial difference. Repeating readings on the same specimen can reveal inconsistency, but repeated readings are not additional independent individuals.
Interpret repeat readings and their limits
Suppose the same pod is read as 51, 52 and 51 mm. Their sum is 154 mm and the mean is 154 divided by three, approximately 51.3 mm. Averaging summarises the repeats and may reduce some random reading variation. It does not change the ruler’s whole-mm divisions into a proven tenth-mm measuring instrument, remove a consistently wrong endpoint or establish a genotype. State the original instrument resolution and the reporting rule.
The invented mass sample 1.8, 2.1, 2.2, 2.2, 2.4, 2.5, 2.7 and 2.8 g spans one gram. Its mean is 18.7 divided by eight, 2.3375 g, which can be reported as about 2.34 g when a task requests two decimal places. That calculated reporting precision does not prove matching raw measurement accuracy. These written protocols and datasets develop investigation reasoning; actual instrument use and practical competence still require supervised evaluation.
Sources for this lesson
Pause and explain
Three repeat readings of one pod represent how many independent pods?
Worked example
Summarise three repeat length readings of one pod: 51, 52 and 51 mm.
Show the worked solution
- Add 51 + 52 + 51 = 154 mm and divide by three.
- Report approximately 51.3 mm while retaining one specimen and the ruler’s actual resolution.
Answer The mean repeat reading is approximately 51.3 mm. This is one pod measured three times, not three independent pods.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find the mean of 51, 52 and 51 mm for one pod, to one decimal place.
Give me a hint
Add all repeat readings before dividing by three.
Compare my reasoning
- Sum the readings to 154 mm.
- Divide 154 by 3 and round the calculation to one decimal place.
Approximately 51.3 mm; the record still represents one pod.
Look for these in your work
- Correct mean and mm
- One specimen distinguished from repeats
For mass readings 1.8, 2.1, 2.2, 2.2, 2.4, 2.5, 2.7 and 2.8 g, find the range and mean to two decimal places.
Give me a hint
Use one unit and the eight-individual denominator.
Compare my reasoning
- Range is 2.8 − 1.8 = 1.0 g.
- The sum is 18.7 g; divide by 8 to get 2.3375 g and round.
Range 1.0 g; mean approximately 2.34 g. Reporting the mean to two decimals does not prove the balance measured to that accuracy.
Look for these in your work
- Correct range and mean
- Precision limit
One observer follows curved pod surfaces while another uses straight endpoint distance. Explain how to repair the investigation.
Give me a hint
Variation in method can masquerade as variation in the feature.
Compare my reasoning
- The two protocols can yield different lengths on the same specimen.
- Choose and document one appropriate endpoint/method rule, use it consistently and retain repeat readings.
Use one stated method throughout, with the same units, comparable specimens and repeats. The mixed readings cannot safely be treated as biological differences alone.
Look for these in your work
- Method confounding
- Consistent corrected protocol
Common mistakes
- Counting repeats as additional individuals
- Mixing g and mm
- Claiming an average upgrades instrument resolution
What can you explain now?
Three repeat readings of one pod represent how many independent pods?
Compare with the explanation
One
Repeats improve information about the same specimen.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, plan comparable body measurements and interpret repeat readings and their limits. Keep units, the stated sample and the limits of the evidence clear.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.