How do people judge the sizes of things? What determines people’s evaluations of quantities such as prices or wages? People’s judgements and evaluations are typically relative; the same quantity will be judged or evaluated differently when it appears in different comparison samples. This chapter describes a simple psychological account – the Decision by Sampling model – of how sample-based judgements and evaluations are made. According to the model, what matters is the relative ranked position of an item within a comparison sample. For example, an income of $50,000 a year will be evaluated more favourably within a context of four lower and two higher incomes than in relation to one higher and five lower incomes. According to Decision by Sampling, estimates of the relative ranked position of items within comparison contexts are made by simple sampling and ordinal comparison processes. These estimates are assumed to underpin choice and valuation. The chapter reviews the Decision by Sampling model, relates it to other models such as Adaptation Level Theory and Range Frequency Theory, and shows how it can explain the shape of utility curves and probability weighting functions. The relation of coding efficiency to rank-based models is also discussed.