This is a complete unit to introduce the use of algebraic models (linear and simple exponential) to describe deterministic and stochastic change, as it is seen in the world around us. The unit approaches the fitting of models to bivariate data via the underlying properties of constant additive or multiplicative change and encourages students to critique the ‘fit’ of algebraic models prior to their refinement and use. The unit contains a wealth of data, drawn from a range of aspects of the modern world. Extensive notes are provided on the use of ClassPad technology, to refine a user defined algebraic model as well as to obtain models of best fit. The accompanying .vcp file contains all the data that is used in the unit, in a Spreadsheet and as Statistics variables.