What you need to know
- Unit 6Comment on the number of readings, the range of measurements, and significant figures, including rounding a value that claims too much precision.
You are usually commenting on a method or a table you are shown. Do not turn a comment about figures into a significance test. There is no rule that every table must have the same number of significant figures.
By the end, you should be able to…
- Judge whether there are enough readings, and enough range, for the claim.
- Separate repeats at one value from a range of values.
- Round a processed number back to the precision of the measurements.
Build your understanding
Number and range answer different doubts
Two temperatures can show that those two results differ. They cannot show an optimum or a plateau. An illustrative series at 25 °C and 27 °C has neither the number of values nor the range to support “the optimum is 26 °C.”
Five values spread from 10 °C to 50 °C can suggest a rise and a fall, but one reading at each temperature cannot show whether a point is reliable. Repeats at one temperature describe scatter at that temperature. They do not map temperature. Say which of the two is missing.
A narrow cluster is not a wide range
The range is the span from the lowest to the highest value. The interval is the gap between neighbours. Five temperatures from 30 °C to 34 °C are numerous and still narrow. They cannot support a conclusion about an enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. from 0 °C to 80 °C.
A wide range can still miss the region that matters. If the claim is about an optimum between 30 °C and 40 °C, and the table jumps from 20 °C to 50 °C, say that intermediate values are needed. Do not praise a range only because the numbers are round.
Figures that the instrument did not give
Comment when a table pretends to more precision than the readings. Illustrative times of 40 s, 41 s and 44 s have the mean (40 + 41 + 44) ÷ 3 = 125 ÷ 3 = 41.666… s. Those clocks were read to the nearest second, so 41.6667 s is not an honest record. 42 s to the nearest second, or 41.7 s if one extra decimal place is kept while the division is shown, matches the measurements better.
When you amend the figure, state the rule. Adding and averaging is limited by decimal places. Multiplying and dividing is limited by the least precise number of significant figures. Do not “round up” every digit as if the instruction meant make the number larger. 41.2 s to the nearest second is 41 s, not 42 s.
Make your reasoning visible
Quote the table
“Only 25 °C and 27 °C were used” is a comment. “They needed more results” could fit any experiment.
Show the rounding
Write the unrounded mean, then the value you would record, and say nearest second or one decimal place.
Avoid this misconception
The claim: More readings always means a better range.
The correction: Readings can be numerous and still sit in a narrow band. Range is the span. Repeats are extra trials at a value.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Illustrative times are 40 s, 41 s and 44 s. A table gives the mean as 41.6667 s. What should be recorded, to the nearest second, and why?
Show suggested answer
125 ÷ 3 = 41.666… s, which rounds to 42 s to the nearest second. The clocks were read in whole seconds, so four decimal places claim a precision the readings do not have.
Practice question
Retrieval and explanation · suggested answer
An enzyme rate is measured only at 30 °C, with five repeats. Can the student conclude how temperature affects the rate?
Show suggested answer
No. The repeats describe scatter at 30 °C. There is no range of temperature, so there is no evidence of an effect of temperature.
Practice question
Retrieval and explanation · suggested answer
Distances of 10, 20, 30, 40 and 50 cm are used, one reading each, and the conclusion says the rate is inversely proportional to distance. What can you support?
Show suggested answer
Five distances can suggest a pattern across that range. One reading at each distance cannot show that a point is reliable, and the distances alone do not prove inverse proportion. That check needs a stated graph, such as rate against 1/distance, plus repeats if reliability is the issue.
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 table uses pH 6.8, 6.9, 7.0, 7.1 and 7.2, three repeats at each, and concludes that the enzyme has no optimum near neutral pH because the means are similar. What can the range support, and what figure problem would you look for in the means? Assume the times were whole seconds.
Show a hint
Similar means inside a tiny interval are not a map of pH.
Explore the reasoning
The data can support a comparison inside pH 6.8 to 7.2 only. They cannot show that there is no optimum elsewhere, because the range never leaves that narrow band. Means of whole-second times should not be written with a long string of decimal places. The assumption is that temperature, enzyme and substrate were kept the same.
Recall this lesson
Lesson recall questions
Question 1 of 5: A gas-syringe reading of 9.8 cm³ sits far above 4.1, 4.0, 4.2 and 3.9 cm³ of oxygen at the same light intensity. Outline how to treat it.
Unit 6 implementation and measurements
Practice question
Retrieval and explanation · suggested answer
A gas-syringe reading of 9.8 cm³ sits far above 4.1, 4.0, 4.2 and 3.9 cm³ of oxygen at the same light intensity. Outline how to treat it.
Lesson: I2: Wrong units and a point that does not fit
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Extra digits are not extra information
I1 · Readings, range and significant figuresNIST’s guide to the SI includes rules for expressing the value of a quantity, units included. A school table is not that publication. The transferable rule is smaller:…Specification Unit 6 · Implementation and measurementsRead & use the scienceClose article
Why this matters
NIST’s guide to the SI includes rules for expressing the value of a quantity, units included. A school table is not that publication. The transferable rule is smaller: do not print digits the measuring instrument did not justify. A long calculator display is a calculation, not a new observation. A laboratory scientist records the instrument's resolution, repeats and variation so a reader can judge what digits are defensible. Rounding a mean too early can discard information, but printing ten decimal places after reading a ruler to millimetres pretends certainty. Show the working with adequate digits, then report a result at a precision supported by the measurements.
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 · Implementation and measurements Evaluate readings and methods: range and number of measurements, significant figures, units, inconsistent points and realistic improvements.
Use the science
A mean of whole-second times is printed as 41.6667 s. What information do the extra digits seem to add, and why is that misleading?
Explore a possible response
They look like a measurement to ten-thousandths of a second. The clocks were read to the nearest second, so those digits are an artefact of the division, not a finer observation.