WJEC A Level Further Statistics A — Unit Papers & Mark Schemes

Recognising which probability distribution to apply is the essential first step in most questions. The Poisson distribution models events occurring independently at a constant average rate; the geometric distribution models the number of trials until the first success; the exponential distribution is the continuous analogue of the Poisson for waiting times. Practise identifying the distribution from the problem context before computing probabilities. Hypothesis testing questions demand a structured approach: state the null and alternative hypotheses, identify the test statistic and its distribution under H₀, calculate the test statistic from the data, compare with the critical value or compute the p-value, and state your conclusion in context. Marks are awarded for each step, so omitting the formal statement of hypotheses or the contextual conclusion costs marks unnecessarily. Chi-squared test questions require you to calculate expected frequencies, combine cells where expected frequencies fall below 5, compute the test statistic, and determine degrees of freedom correctly. Practise computing degrees of freedom for goodness-of-fit tests (categories minus 1 minus estimated parameters) and contingency tables ((rows – 1)(columns – 1)).