Use the probability scale from 0 to 1, calculate probabilities of equally likely outcomes using sample spaces and listings, and apply the rules for mutually exclusive and exhaustive events including combined events in two-way tables.

What this dot point is asking

This is the foundation of WJEC probability. You are asked to use the probability scale from $0$ to $1$, to calculate the probability of equally likely outcomes by counting (using sample space diagrams and systematic listing), and to apply the rules for mutually exclusive and exhaustive events, including combined events shown in two-way tables. The central idea is that probability is a fraction of favourable outcomes over total outcomes, and that the probabilities of all outcomes sum to $1$. It is examined on both components at every tier.

The probability scale

Every probability sits between $0$ and $1$.

So drawing a particular card from a pack of $52$ has probability $\tfrac{1}{52}$, and drawing any heart has probability $\tfrac{13}{52} = \tfrac{1}{4}$.

The complement and exhaustive events

The probabilities of all possible outcomes add to $1$.

Because something must happen, the probabilities of all the possible outcomes sum to $1$. Events that together cover every possibility are exhaustive. The complement of event A (the event "A does not happen") therefore has probability $P(\text{not A}) = 1 - P(A)$. This is the fastest route to "find the probability it is not ..." questions: subtract from $1$ rather than adding many cases.

Mutually exclusive events

Some events cannot occur at the same time.

So the probability of rolling a $1$ or a $2$ on a fair die is $\tfrac{1}{6} + \tfrac{1}{6} = \tfrac{1}{3}$. The "add for or" rule only applies when the events cannot overlap.

Sample spaces and combined events

Listing all outcomes makes combined events countable.

A two-way table organises combined data so probabilities can be read off directly from the totals.

Why this matters

Probability basics underpin every later topic in the strand: tree diagrams, Venn diagrams and expected outcomes all build on the scale, the complement rule and systematic counting. The marks reward writing probabilities correctly (as fractions, not "$6$ out of $36$" left unsimplified), using $1 - P(A)$ efficiently, and listing combined outcomes without missing any. Recognising mutually exclusive events and applying the "add for or" rule correctly is a common discriminator, and a sample space or two-way table is the reliable tool for combined events.