Statistics in a Nutshell

Chapter 4. Descriptive Statistics and Graphics

Most of this book, as in most statistics books, is concerned with statistical inference, which is the practice of drawing conclusions about a population using statistics calculated on a sample considered to be representative of that population. However, this particular chapter is concerned with descriptive statistics, meaning the use of statistical and graphic techniques to present information about the data set being studied. Computing descriptive statistics and examining graphic displays of data is an advisable preliminary step in data analysis. You can never be too familiar with your data, and the time you spend examining the actual distribution of the data collected (as opposed to the distribution you expected it to assume) is always time well spent. Descriptive statistics and graphic displays are also the final product in some contexts: for instance, a business may want to monitor total volume of sales for its different locations without any desire to use that information to make inferences about other businesses.

Populations and Samples

The same data set may be considered as either a population or a sample, depending on the reason for its collection and analysis. For instance, the final exam grades of the students in a class are a population if the purpose of the analysis is to describe the distribution of scores in that class. They are a sample if the purpose of the analysis is to make some inference from those scores to the scores ...