The big idea: Paper 1B and the long Paper 2 data question are built on one skill: getting information out of data — a line graph, a bar chart, a stacked bar, a box plot or a table.
There is nothing to memorise (biology has no data booklet). Instead the examiner gives you a display and asks you to do one of a small set of jobs:
Read off a value · estimate a value between points · describe a trend · compare two series · predict beyond the data.
Get the verb right and you get the mark — each verb has a fixed way to answer.
- Variable
- Something measured or changed. The x-axis is usually what was changed (independent); the y-axis is what was measured (dependent).
- Trend
- The overall way the y-value changes as the x-value changes — e.g. 'rises then levels off'.
- Read off (a value)
- Take a single value straight from the data at a stated point — and always quote its unit.
- Interpolate
- Estimate a value BETWEEN two points you do have — usually safe, because it stays inside the data.
- Extrapolate
- Predict a value BEYOND the data you have — less reliable, because the trend might not continue.
Always carry the unit and the axis: A number on its own scores nothing. '42' is wrong; '42 µmol min⁻¹' is right.
Before you read, check which axis is which and what the scale steps are (is each gridline 1, 2, 5 or 10?). Misreading the scale is the most common way to lose easy marks.
Most biology curves have the same shape: the rate rises steeply, then levels off to a plateau once some other factor becomes limiting.
Work through the four reading jobs on exactly this kind of curve.
A typical biology curve: the rate RISES steeply, then levels off to a PLATEAU. Reading it means (1) reading off a value at a given point, (2) describing the trend in two parts — the rise then the plateau — and (3) knowing you may interpolate between points but should be cautious extrapolating past the plateau.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
1 · Read off a value
- Find the x-value you are asked about along the bottom axis.
- Go up to the curve, then across to the y-axis.
- Read the height and write it with its unit — e.g. at this light intensity the rate is 30 µmol min⁻¹.
2 · Describe the trend
- Give the overall direction first: as light intensity increases, the rate increases.
- Then name the change of pattern: …then levels off / plateaus.
- Support it with figures: the rate rises steeply up to ~X, then stays roughly constant at ~Y. (Two parts = two marks.)
Interpolate vs extrapolate: Interpolate = read between points you have. If the curve passes through (2, 30) and (4, 42), a value at x = 3 of about 36 µmol min⁻¹ is a safe interpolation.
Extrapolate = continue the line past the last point. Because the curve has plateaued, predicting that the rate at x = 9 is still about 44 µmol min⁻¹ is reasonable — but say it is a prediction, since the trend could change.
Estimating at from neighbours: .
To read off a value: find the point on the curve above the x-value you want, run straight across to the y-axis, and read the height — always with its unit. Between two plotted points you INTERPOLATE; beyond the last point you EXTRAPOLATE (a prediction, not a measurement).
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Worked reading: Take the photosynthesis-style curve above.
Read off — at a low light intensity the point sits low on the rising part, so the rate is small (say 15 µmol min⁻¹).
Describe the trend — as light intensity increases the rate increases steeply, then levels off to a plateau where increasing the light no longer raises the rate.
Extrapolate — past the plateau the rate stays roughly constant, because another factor (CO₂ or temperature), not light, now limits it.
| What the question asks | What it means | How to answer |
|---|---|---|
| Read off / State / Identify | Give a single value or label straight from the data | Find the point, drop to the axis, read the value — INCLUDE the unit |
| Estimate | Give a value BETWEEN two plotted points (interpolate) | Read between the gridlines; a sensible in-between value is fine |
| Describe the trend / relationship | Say in words how y changes as x changes | Direction first (rises/falls), then any change of pattern (plateau, peak), with support figures |
| Compare and contrast / Distinguish | Put two or more data series side by side | Give at least one SIMILARITY and one DIFFERENCE, each backed by a figure |
| Predict | Give a value or trend BEYOND the data (extrapolate) | Continue the trend, state a value, and give a REASON from the pattern |
Learn what examiners really want
See exactly what to write to score full marks. Our AI shows you model answers and the key phrases examiners look for.
How this is tested: This is the most-tested data skill in the course — the spine of Paper 1B and the first long question of Paper 2.
Typical 1-mark items: read off / state a value, identify the largest bar or the best treatment from a table, estimate a value between points.
Typical 2–3 mark items: describe a relationship (two parts: direction + plateau/peak, with figures), compare and contrast two series (one similarity AND one difference, each with a figure), or predict a value beyond the data with a reason.
Golden rule: answer only from the data. Never add a mechanism the graph does not show — and never give a number without its unit.
A class measured the rate of reaction of two enzymes, P and Q, at different substrate concentrations (everything else kept constant). Their results:
| Substrate conc. (g dm⁻³) | Enzyme P rate (µmol min⁻¹) | Enzyme Q rate (µmol min⁻¹) |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 18 | 9 |
| 2 | 30 | 16 |
| 3 | 38 | 22 |
| 4 | 42 | 27 |
| 5 | 44 | 31 |
| 6 | 44 | 34 |
| 7 | 44 | 36 |
IB-style question — read, describe, compare and predict from the data
(a) State enzyme P's rate at a substrate concentration of 2 g dm⁻³. [1]
(b) Describe the relationship between substrate concentration and enzyme P's rate. [2]
(c) Compare and contrast the rates of enzymes P and Q across the range tested. [2]
(d) Predict enzyme Q's rate at 8 g dm⁻³, and give a reason. [2]
How to score all seven marks
- (a) Read off. Find the row for 2 g dm⁻³ and read P's column: the rate is 30 µmol min⁻¹. (Quoting the unit is part of the mark.)
- (b) Describe — direction first. As substrate concentration increases, P's rate increases (0 → 18 → 30 → 38 µmol min⁻¹). Mark 1.
- (b) Describe — change of pattern. …then levels off / plateaus at about 44 µmol min⁻¹ (the value stops rising from 5 g dm⁻³ onward). Mark 2. Two parts, two marks.
- (c) Compare — a SIMILARITY. Both enzymes' rates increase with substrate concentration and then plateau — both follow the same overall shape. Mark 1.
- (c) Contrast — a DIFFERENCE (with figures). P's rate is higher and plateaus sooner: at 7 g dm⁻³, P = 44 but Q = 36 µmol min⁻¹; P levels off by ~5 g dm⁻³ while Q is still rising. Mark 2.
- (d) Predict + reason. Q is still climbing (34 → 36) and rising more slowly, so at 8 g dm⁻³ it is about 37–38 µmol min⁻¹ — just above the last value, because Q is approaching its own plateau. The value AND the reason are needed for both marks.
Final answer
(a) 30 µmol min⁻¹. (b) As substrate concentration rises, P's rate rises, then plateaus at ~44 µmol min⁻¹. (c) Similarity: both rise then plateau. Difference: P is higher and plateaus earlier (P=44 vs Q=36 at 7 g dm⁻³; Q still rising). (d) ~37–38 µmol min⁻¹ — Q is still rising but slowing, approaching its plateau.
✓ Why this scores full marks: Every answer is read straight from the data and every value carries its unit. The describe answer has two parts (rise + plateau), the compare answer gives a similarity AND a difference each with a supporting figure, and the predict answer gives a value plus a reason.
The classic way to lose marks here is to give only one half — only the rise (not the plateau), or only a difference (not a similarity) — or to predict without a reason.