# 8: Descriptive Statistics

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Statistics are often presented in an effort to add credibility to an argument or advice. You can see this by paying attention to television advertisements. Many of the numbers thrown about in this way do not represent careful statistical analysis. They can be misleading, and push you into decisions that you might find cause to regret. These chapters will help you learn statistical essentials. It will make you into an intelligent consumer of statistical claims.

• 8.1: Introduction
Like most people, you probably feel that it is important to "take control of your life." But what does this mean? Partly it means being able to properly evaluate the data and claims that bombard you every day. If you cannot distinguish good from faulty reasoning, then you are vulnerable to manipulation and to decisions that are not in your best interest. Statistics provides tools that you need in order to react intelligently to information you hear or read.
• 8.2: Populations and Samples
Before we begin gathering and analyzing data we need to characterize the population we are studying.
• 8.3: Categorizing data
Once we have gathered data, we might wish to classify it. Roughly speaking, data can be classified as categorical data or quantitative data.
• 8.4: Sampling methods
The first thing we should do before conducting a survey is to identify the population that we want to study.
• 8.5: How to mess things up before you start
There are number of ways that a study can be ruined before you even start collecting data. The first we have already explored – sampling or selection bias, which is when the sample is not representative of the population. One example of this is voluntary response bias, which is bias introduced by only collecting data from those who volunteer to participate. This is not the only potential source of bias.
• 8.6: Experiments
So far, we have primarily discussed observational studies – studies in which conclusions would be drawn from observations of a sample or the population. In contrast, it is common to use experiments when exploring how subjects react to an outside influence. In an experiment, some kind of treatment is applied to the subjects and the results are measured and recorded.
• 8.7: Exercise
• 8.8: Presenting Categorical Data Graphically
Categorical, or qualitative, data are pieces of information that allow us to classify the objects under investigation into various categories.
• 8.9: Presenting Quantitative Data Graphically
• 8.10: Numerical Summaries of Data
• 8.11: Measures of Central Tendency
• 8.12: Measures of Variation
In addition to the mean and median, which are measures of the "typical" or "middle" value, we also need a measure of how "spread out" or varied each data set is. There are several ways to measure this "spread" of the data.
• 8.13: Exercises

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