ABSTRACT
Distance measurement is much less direct, and is based on the 1912 discovery, by Henrietta Leavitt, of a relationship between the period of a certain class of variable stars, known as the Cepheids, and their luminosity. The intensity of Cepheids varies regularly with a period of between 1.5 and something over 50 days, and the mean intensity increases predictably with period. The chapter discusses statistical methods for a simple linear model of the form, allowing the key concepts of linear modelling to be introduced without the distraction of any mathematical difficulty. The least squares calculations derived are available as part of the statistical package and environment R. The function lm fits linear models to data, including the simple example currently under consideration. Hypothesis testing is useful when there are good reasons to stick with some null hypothesis until there are compelling grounds to reject it.
