122 146 65 162 148 155 136 151 151 153 201 156 235 157 160 171 178 197 142 131This is a data set of 20 observations. The number of items in a sample is called the sample size . We often denote the sample size by

Next suppose we ask the students their hair color and get the responses:

Red Blond Blond Brown Brown Red Blond Blond Brown Black Blond Red Red Brown Black Brown Red Black Brown BlondThis is another data set of 20 observations.

Often our data set is a **sample** of observations from some reference
** **.
For example, the 20 weights might be sample of the weights of 20 students
from a university. We might want to **infer** something about the weights
of the population based on this sample. These are problems of statistical
inference which we will take up in later chapters. In this chapter, though,
we just want to discuss ways for describing data sets.

To begin with, basically data come in two types: **discrete ** and
**continuous **.
Discrete data have natural categories while continuous data do not. The
hair color data set is discrete while the weights are continuous. We will
treat discrete data first and then continuous data.