Statistics and Probability
AA SL Topic 4 (4.1–4.9)
This topic almost never asks you to derive anything from scratch — it tests whether you can look at a scenario, pick the right model (raw data? grouped data? binomial? normal? bivariate?), feed the right numbers into your GDC, and then interpret the output in the context you were given. Marks are lost far more often on interpretation and rounding than on the arithmetic itself, so treat every 'comment' or 'justify' part as worth exactly as much attention as the calculation before it.
Overview: How This Topic Is Actually Examined
What this topic really tests
Statistics and Probability at SL splits into three jobs: describing one data set (mean, spread, shape), modelling chance (probability rules, binomial, normal), and relating two variables from a sample (correlation, regression). Every exam question is one of these three jobs wearing a different real-world costume — waiting-room ages, tablet dissolution times, factory defect rates, ice-cream sales.
- Almost everything is calculator-active: you're expected to use 1-Var/2-Var Stats, binomialpdf/cdf, and normalcdf/inverse normal fluently.
- The 'hard' marks sit in the reasoning parts — explaining why a mean changes, why an extrapolation is unreliable, why two events can't be both independent and mutually exclusive.
- AA SL has no formal hypothesis testing or confidence intervals — 'inference' at this level means using correlation and regression to describe and predict, nothing more.
The shape of the chapter
Command terms that decide how you write the answer
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Write down | State a result straight from the GDC or given data, no working needed. | AO1 | No method marks exist here — a slip is a lost mark outright, so double-check the GDC screen before transcribing. |
| Calculate / Find | Produce the correct value; working should be evident (formula or GDC inputs). | AO2 | Quote the values you entered into the GDC (n, p, μ, σ) — if the final answer is wrong, this earns the method mark. |
| Show that | Prove a stated value using full working, not just the GDC answer. | AO2 | Quoting only the given answer scores 0 — you must display enough intermediate arithmetic to justify it. |
| Determine | Same weight as 'Calculate' but often used when a decision or comparison follows. | AO2 | If a comparison follows, the numerical answer alone isn't enough — the follow-up reasoning carries its own mark. |
| Comment on / Justify | Give a reasoned statement tied to the context of the scenario. | AO3 | The mark sits on the CONTEXT sentence, not the number — a bare 'yes' or 'the SD increased' with no reference to the scenario scores 0. |
Key point
Overview