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How to Judge Statistics Questions – Edexcel International A Level (IAL)
📊 How to Judge a Statistics Question
Edexcel International A Level (IAL – S1 / S2)
Use this as a mental checklist before solving ANY question
1️⃣ Identify the Data Format (Critical First Step)
Ask immediately: Is the data
- Raw data?
- Frequency table?
- Grouped data?
- Summary statistics given:
- n
- mean (x̄)
- ∑x
- ∑x²
⚠️ Edexcel trap: Students often use raw-data formulas when only summary statistics are given.
2️⃣ Check if Data is Grouped
- ✔ Use midpoints
- ✔ Answers are estimates
- ✔ Always state the word “estimate”
❌ Never use exact formulas for grouped data.
3️⃣ Spot the Key Command Word
| Command Word | What Edexcel Expects |
|---|---|
| Show that | Full algebra leading to the given answer |
| Hence | Use previous result directly |
| Without further calculation | Reasoning only – no numbers |
| Find | Clear method + final value |
| Estimate | Midpoints + explanation |
| Comment / Compare | Centre + spread |
| State | Short, correct sentence |
4️⃣ Transformation Rules (Heavily Tested)
| Transformation | Mean | Variance | SD |
|---|---|---|---|
| X + a | + a | No change | No change |
| X − a | − a | No change | No change |
| aX | × a | × a² | × a |
⚠️ Common Edexcel trap: Students change the standard deviation when adding a constant.
5️⃣ Combined Data → Weighted Thinking Only
Never average averages.
x̄ = (n₁x̄₁ + n₂x̄₂) ÷ (n₁ + n₂)
Same logic applies to:
- Total ∑x
- Total ∑x²
📌 Examiner comment: “Candidate failed to weight correctly.”
6️⃣ Formula Discipline (IAL Marking Is Strict)
Variance = (∑x² ÷ n) − (x̄)²
- ✔ Method marks depend on writing the formula
- ❌ Jumping straight to calculator loses marks
7️⃣ Sample vs Population
Always read the question carefully.
Using the wrong denominator = automatic mark loss.
8️⃣ Probability Questions – Identify Structure First
- Independent or dependent?
- With replacement or without?
- Tree diagram or direct probability?
⚠️ Edexcel favourite trap: assuming independence when events are dependent.
9️⃣ Comparison Questions
A correct Edexcel comparison must include:
- Centre (mean or median)
- Spread (variance, SD or IQR)
❌ “Group A is better”
✅ “Group A has a higher mean and lower standard deviation, so results are more consistent.”
✅ “Group A has a higher mean and lower standard deviation, so results are more consistent.”
🔟 Rounding & Accuracy
- Follow instructions exactly
- If none given:
- Mean → 3 significant figures
- SD → 3 significant figures
⚠️ Rounding too early leads to incorrect final answers.
🧩 Golden Edexcel Rule
Shifts move averages.
Scales stretch spread.
Scales stretch spread.