2 - Variation

Summary

The key idea in statistical modelling is to treat the data as the outcome of a random experiment. The purpose of this chapter is to understand some consequences of this: how to summarize and display different aspects of random data, and how to use results of probability theory to appreciate the variation due to this randomness. We outline the elementary notions of statistics and parameters, and then describe how data and statistics derived from them vary under sampling from statistical models. Many quantities used in practice are based on averages or on ordered sample values, and these receive special attention. The final section reviews moments and cumulants, which will be useful in later chapters.

Statistics and Sampling Variation

Data summaries

The most basic element of data is a single observation, y — usually a number, but perhaps a letter, curve, or image. Throughout this book we shall assume that whatever their original form, the data can be recoded as numbers. We shall mostly suppose that single observations are scalar, though sometimes they are vectors or matrices.

We generally deal with an ensemble of n observations, y₁, …, y_n, known as a sample. Occasionally interest centres on the given sample alone, and if n is not tiny it will be useful to summarize the data in terms of a few numbers. We say that a quantity s = s(y₁, …, y_n) that can be calculated from y₁, …, y_n is a statistic.