What you need to know
- Unit 6Identify and amend incorrect units, and check a reading that is inconsistent with the others, including a point off a line.
Amending a unit is not the same as deleting a point. An inconsistent reading is checked before it is left out. This is not a significance test.
By the end, you should be able to…
- Find a unit that contradicts the instrument or the quantity.
- Amend the heading or convert the numbers, and say which you did.
- Check an inconsistent reading or a point off the line before rejecting it.
Build your understanding
The unit has to be the one you measured
A wrong unit is a comment in its own right. Illustrative syringe volumes of 8.5, 9.0 and 8.0 recorded under the heading “Volume / mm³” do not match a school syringe graduated in cm³. The amendment is to head the column cm³, unless the student really did convert and can show the factor.
A rate heading of “per minute” with no quantity is also unfinished. Gas collected by a syringe needs a volume unit as well, such as cm³ min⁻¹. Leaving the unit only in the first cell, or mixing cm³ and mm³ in one column, makes the later graph impossible to trust.
Convert, or change the heading, but do not do both silently
If the heading is wrong and the numbers match the instrument, change the heading and leave the numbers. If the heading is the unit you must use and the numbers are in another unit, convert every number and show the factor. Doing neither, or halving some rows only, leaves a column that cannot be averaged.
Illustrative leaf lengths written as 12.5 under the heading “/ m”, measured with a centimetre ruler, are not 12.5 m leaves. Amend the heading to cm, or convert 12.5 cm to 0.125 m, and use the same choice for the whole column. Then recalculate any mean that used the mixed column.
A point off the line is checked, not hidden
Compare a reading with the other repeats or with the pattern on the graph. Illustrative titres 9.20, 9.40, 9.30 and 14.10 cm³ make 14.10 cm³ the inconsistent value. A graph point far from an otherwise steady line is the same idea.
Check the scale, the unit and the copied row, and repeat that trial if the material is still available. If the repeat agrees with the cluster, you can leave the original out of the mean and say so. If you cannot repeat it, do not silently delete a point the examiner can see. Small differences among 9.20, 9.40 and 9.30 are ordinary scatter, not four anomalies.
Make your reasoning visible
Heading first
Read the unit before you comment on the size of the number. A huge leaf or a tiny gas volume is often a unit error.
Point at the odd value
Name 14.10 cm³, or the coordinate off the line. Then say the check, not only the word anomaly.
Avoid this misconception
The claim: Any result you dislike should be crossed out, and the unit can be fixed later.
The correction: Check a suspect reading against the others and against the unit in the heading. Amend a wrong unit for the whole column. Do not hide a point the table still contains.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
A column is headed “Rate / min” and the cells are 2.5, 3.1 and 2.8, collected with a gas syringe. What is missing or wrong?
Show suggested answer
The volume unit is missing. The heading should state the rate actually measured, for example cm³ min⁻¹. “Per minute” alone does not say what was divided by time.
Practice question
Retrieval and explanation · suggested answer
Titres are 9.20, 9.40, 9.30 and 14.10 cm³. Which value needs checking, and what would you do before using a mean?
Show suggested answer
14.10 cm³ is inconsistent with the other three. Check the burette reading and the record, then repeat that titration if you can. Do not drop 14.10 from the mean until it has been checked. If a repeat matches the cluster, exclude the original and say so.
Practice question
Retrieval and explanation · suggested answer
Lengths measured in centimetres are entered under the heading “Length / m” with no conversion. How do you amend the table?
Show suggested answer
Either change the heading to Length / cm and keep the numbers, or convert every length to metres and show the factor, such as dividing centimetres by 100. Do not convert only the mean.
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 graph of rate against temperature rises smoothly except for one point far below the line. The table’s rate heading has no unit, and the student redraws the line through the odd point. Give two distinct comments.
Show a hint
One comment is about the unit. The other is about the point.
Explore the reasoning
The rate column or axis needs a unit before the scale can be interpreted. The odd point should be checked against the table and repeated if possible. Redrawing the line through one unchecked point lets that point dictate the relationship. The assumption is that the other points really do follow one pattern.
Recall this lesson
Lesson recall questions
Question 1 of 5: A table of pondweed oxygen volumes is headed ‘volume (cm³)’ but the fictional values are 12.4 mm of bubble. Identify the error and amend it.
Unit 6 implementation and measurements
Practice question
Retrieval and explanation · suggested answer
A table of pondweed oxygen volumes is headed ‘volume (cm³)’ but the fictional values are 12.4 mm of bubble. Identify the error and amend it.
Lesson: I3: Improvements that reduce a named error
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A quantity without a unit is unfinished
I2 · Wrong units and a point that does not fitNational measurement laboratories publish the SI so that a result can be compared in a stated unit. School data are not a standards-laboratory report. Two numbers can still…Specification Unit 6 · Implementation and measurementsRead & use the scienceClose article
Why this matters
National measurement laboratories publish the SI so that a result can be compared in a stated unit. School data are not a standards-laboratory report. Two numbers can still not be compared until the unit is the one that was measured. A number can be copied perfectly and still be unusable if its unit is wrong. Unit conversion is a scientific operation, not a cosmetic label change. Before plotting or comparing two studies, check whether mass is in grams or kilograms and whether a rate means per second or per minute. A tenfold or thousandfold mistake can overwhelm a real biological effect while leaving a neat-looking graph.
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
Two students both record 12.5 for a leaf. Why might those records still disagree?
Explore a possible response
The unit may differ. 12.5 cm and 12.5 m are not the same length. The heading has to name the unit that matches the ruler, or the numbers have to be converted.
Explore the source
The Mars probe lost in a unit conversion
I2 · Wrong units and a point that does not fitNASA Jet Propulsion Laboratory describes the loss of the Mars Climate Orbiter after a failure to translate English units into metric units in navigation data. Spacecraft engineering is…Specification Unit 6 · Implementation and measurementsRead & use the scienceClose article
Why this matters
NASA Jet Propulsion Laboratory describes the loss of the Mars Climate Orbiter after a failure to translate English units into metric units in navigation data. Spacecraft engineering is far beyond a Biology practical, but the lesson is exactly why Unit 6 asks you to amend incorrect units. A value without a unit cannot be checked, and a mislabeled rate may change a result by a factor of 60 or 1000. Put the unit in the table heading, convert explicitly and then use the converted value throughout the calculation and graph.
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 gas rate is recorded as 12 cm³ min⁻¹ but labelled 12 cm³ s⁻¹. By what factor is the labelled rate wrong?
Explore a possible response
One minute contains 60 seconds. The true rate is 12/60 = 0.20 cm³ s⁻¹, so the wrongly labelled 12 cm³ s⁻¹ is 60 times too high.
Explore the source
- Mars Climate Orbiter missionNASA Jet Propulsion Laboratory