Mathematical and statistical skills

Use SI units and prefixes consistently, convert scales such as kilo, milli, micro and nano, and interpret inequality or proportional symbols.

Write the unit with the number and use standard form for very large or small biological quantities.

2.5 µm is 2.5×10⁻⁶ m, not 2.5×10⁻³ m; the prefix determines the power of ten.

A correct numerical value with a mismatched unit is still an incorrect biological result.

Estimate the order of magnitude first, then calculate and report a precision justified by the measured data.

Keep extra digits during working but round the final quantity consistently.

If the least precise input has two significant figures, a calculated rate should not be reported to six unexplained figures.

Calculator precision is not measurement precision.

Magnification = image size ÷ actual size; use the same units before calculating and apply the relevant area, perimeter or volume formula.

Check whether the question asks for a length, area or volume because scaling powers differ.

A 20 µm cell shown as 40 mm long has magnification 2000× after converting 40 mm to 40 000 µm.

Do not divide by magnification without first matching units.

Use the statistic or percentage that matches the question, and express experimental error relative to the measured quantity.

State the denominator for a percentage change and keep ratios in a comparable form.

A 2-unit change from 10 is 20%, while the same change from 100 is 2%; the absolute change is identical but the relative change differs.

Percentage error is not the same as percentage change between two biological treatments.

Translate between numerical, graphical and algebraic forms; choose an appropriate chart and calculate rate from a gradient or tangent when the relationship changes.

Put the explanatory variable on x, use a meaningful scale and state the interval for any gradient.

The initial tangent to a respiration curve gives the initial rate, while the overall endpoint difference gives an average rate.

A steeper-looking graph may result from unequal axis scales; inspect units and scale before comparing gradients.