What the examiners said about IAL S1 (WST01) — and exactly what to do about it
We read every Pearson Edexcel examiner report for WST01 from June 2024 to June 2025, mapped each question to a topic, and turned the comments into a checklist of the mistakes that cost the most marks.
Summaries are in our own words with links to the originals. Predictions are informed guesses, not a guarantee.
Ranked for January 2027
What to focus on for January 2027
IAL units are sat in January, June and October. The October 2026 series is next; the ranking below is for your next sitting, whichever it is. Ranked by how often each area appeared, how many marks it carried and how weakly it was answered.
Informed prediction, not a guarantee: any topic on the specification can be examined.
1
Probability: Venn, trees, independence
Every series and the largest single block (13–17 marks).
Ordered by how often and how widely they were reported. The first 5 are free in full; Pro unlocks the fix and a worked example for every other one.
#1Standardisation not shown when the question requires it
What examiners saw: The paper states calculator-only solutions are not acceptable for some normal questions. Answers without the z-value line lose the method mark, and some then subtract a correct answer from 1.
Fix: Write P(X < a) = P(Z < (a − μ)/σ) before the probability. Sketch to decide the tail.
Worked example (our own): X ~ N(20, 4²). Find P(X < 26), showing the standardisation.
Loses marks
P(X < 26) = 0.9332 (calculator only)
Earns the marks
P(X < 26) = P(Z < (26 − 20)/4) = P(Z < 1.5)
= 0.9332
Why: If the question says the standardisation must be seen, write the z-value line. Do not then subtract from 1 unless you want the upper tail.
What examiners saw: Fences are calculated but the answer never says which values are outliers (and which are not), or the given quartiles are ignored.
Fix: Calculate both fences, compare the extreme values, and name the outlier(s).
Worked example (our own): Q₁ = 20, Q₃ = 30. The largest value is 48. Outliers are values more than 1.5 × IQR beyond the quartiles. Show that there is exactly one outlier (the next largest value is 41).
#4Histograms: frequency density and reading scales
What examiners saw: Bar heights are read as frequencies, frequency density is used for the wrong bars, and the width/height of a new bar is wrong.
Fix: Area ∝ frequency: frequency = frequency density × width. Find the scale from a bar you know.
Worked example (our own): On a histogram, the bar for 10 ≤ x < 15 has frequency 30. Another bar, 15 ≤ x < 25, has frequency density 2. Find its frequency.
Loses marks
Frequency = 2 (reading the height)
Earns the marks
Frequency density = frequency ÷ class width
15 ≤ x < 25 has width 10, so frequency = 2 × 10 = 20
Why: On a histogram the AREA is frequency. Check the scale using a bar you know.
How often each topic appeared, and for how many marks
Every question in the papers we read, matched to a topic. Numbers are marks (from the published mark schemes; a question that tests two topics has its marks split). Hover or tap a cell to see the questions.
Confidence: high for the series shown — every question was read and mapped by hand. "Series" = how many of the 4 series examined the topic.
Paper by paper
Every paper we analysed, question by question
Topics are ours (matched from each report's question-by-question comments). We don't reproduce the questions: open the official paper from our past-paper page.
A one-page PDF of the ten most common mark-losing mistakes and how to avoid them. Print it and stick it inside your revision folder.
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Sources
Where this comes from
We read the examiner report for every series from June 2024 to June 2025 in full and matched each question to a topic; marks come from the published mark schemes where the per-question totals could be checked against the paper total.
What do examiners say students get wrong in IAL S1 (WST01)?
The most repeated points are: Standardisation not shown when the question requires it; Outliers; Comparisons without figures or context. Each one on this page has the fix and, where free, a worked example showing the wrong and right method.
Which IAL S1 (WST01) topics come up most?
In the material we analysed, the biggest areas were Probability, Venn and tree diagrams, The normal distribution, Location, spread, outliers and coding.
Is this a prediction of the January 2027 paper?
No one outside the exam board knows what will be on the next paper. The "focus" list is an informed prediction from how often topics appeared and how weakly they were answered; it is not a guarantee, so revise the whole specification.
Where does this information come from?
We read 4 Pearson Edexcel examiner reports (June 2024, October 2024, January 2025, June 2025) in full, summarised them in our own words and linked each point to the original.
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