Introduction

Life scientists often collect samples in which the experimental units can be assigned to two or more discrete and mutually exclusive categories. For example, a sample of 20 humans can be partitioned into the two mutually exclusive categories of ‘right-handed’ or ‘left-handed’ (since even people who claim to be ambidextrous still perform a greater proportion of actions with one hand and can be classified as having a dominant right or left hand). These two categories are discrete because there is no intermediate state and mutually exclusive because a person cannot be assigned to both. They also make up the entire set of possible outcomes within the sample and therefore are contingent upon each other, since for a fixed sample size a decrease in the number in one category must be accompanied by an increase in the number in the other and vice versa.

These are nominal scale data (Chapter 3). The questions researchers ask about these data are the sort asked about any sample(s) from a population.

First, you may want to know the probability a sample has been taken from a population having a known or expected proportion within each category. For example, the proportion of left-handed people in the world is close to 0.1 (10%), which can be considered the proportion in the population, since it is from a sample of several million people.