Probability

Probability covers the probability scale, theoretical and experimental probability, sample space diagrams, expected frequency, tree diagrams, Venn diagrams and conditional probability at Higher tier.

The unit rewards drawing rather than reasoning in your head. A sample space diagram turns an awkward two-dice question into simple counting; a tree diagram makes without-replacement problems tractable by forcing you to write each adjusted probability down. Students lose most marks by treating dependent events as independent, forgetting to change both the numerator and the denominator when an item is removed, and adding probabilities when they should multiply. Remember that all outcomes sum to one, and that a probability greater than one always signals an arithmetic error. Answers should be fractions, decimals or percentages, never ratios.