All subjects · Biology · WBI13 Practical Skills in Biology I

Core calculations: means, percentage change, rates and ratios

Edexcel IAL Biology · WBI13 Practical Skills in Biology I

Most Unit 3 maths marks come from a small set of calculations. Lay out your working: method marks can be earned even if the final number is wrong.

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Means and anomalies

Mean = sum of values ÷ number of values.
An anomalous result does not fit the pattern of the others (for example far outside the range of the repeats). Check it by repeating the reading; if it is clearly an error, leave it out of the mean and say so.
Report the mean to the same precision as the data (or one more decimal place).

Percentage change

Percentage change = (final value - initial value) / initial value x 100

A negative answer is a decrease. In CP3-style or osmosis questions, percentage change in mass lets pieces of different starting mass be compared.

The January 2025 report: errors came from dividing by the wrong value. Always divide by the original value.

Rates and ratios

Rate = change ÷ time, for example volume of gas ÷ time, or distance moved by a pollen tube ÷ time. Use the right time interval: a pollen tube measured from hour 1 to hour 5 has grown for 4 hours.
If you measure the time for an end point (colour disappears), rate = 1 / time (units s⁻¹).
Magnification = image size ÷ actual size (same units first).
Ratios: divide by the smallest value, for example 12 : 4 = 3 : 1.

Worked example

A potato cylinder has a mass of 2.50 g before and 2.15 g after soaking. Calculate the percentage change in mass.

Change = 2.15 - 2.50 = -0.35 g
Percentage change = -0.35 / 2.50 x 100 = -14%

Answer: -14% (a 14% decrease)

Key idea: Mean = sum ÷ number (leave out checked anomalies). % change = (final - initial) / initial x 100. Rate = change ÷ time or 1 / time. Magnification = image ÷ actual.

Check yourself

Times for an end point were 42, 45, 44 and 41 s. Calculate the mean time.
Show answer
43 s. (42 + 45 + 44 + 41) / 4 = 43 s.
Repeats were 18, 19, 31 and 17 mm. The 31 mm reading is anomalous. Calculate the mean of the other readings.
Show answer
18 mm. (18 + 19 + 17) / 3 = 18 mm.
A beetroot extract's absorbance rose from 0.24 to 0.66. Calculate the percentage increase.
Show answer
175 %. (0.66 - 0.24) / 0.24 x 100 = 175%.
The blue-black colour disappeared after 40 s. Calculate the rate as 1/time, in s⁻¹.
Show answer
0.025 s⁻¹. 1 / 40 = 0.025 s⁻¹.
A pollen tube was 120 µm long at 1 hour and 480 µm long at 5 hours. Calculate its mean growth rate in µm per hour.
Show answer
90 µm per hour. (480 - 120) / (5 - 1) = 360 / 4 = 90 µm per hour. The January 2024 Unit 6 report notes candidates who divided by 5 instead of 4.
An image of a cell is 36 mm long. The cell is really 12 µm long. Calculate the magnification.
Show answer
x3000. 36 mm = 36 000 µm; 36 000 / 12 = 3000.
Vitamin C content fell from 43.75 to 34.69 arbitrary units. Calculate the percentage decrease to 2 significant figures.
Show answer
21 %. (43.75 - 34.69) / 43.75 x 100 = 20.71, which is 21% to 2 s.f.

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Last updated 8 November 2026. Spotted a mistake? Tell us.