8: Statistics, Collecting Data

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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 data and claims that bombard you every day. If we cannot distinguish good reasoning from faulty reasoning, then we are vulnerable to manipulation and decisions that are not in our best interest. Statistics provides tools that we need in order to react intelligently to information we hear or read. In this sense, statistics is one of the most important things that we can study.

To be more specific, here are some claims that we have heard on several occasions. (We are not saying that each one of these claims is true!)

4 out of 5 dentists recommend Dentyne.
Almost 85% of lung cancers in men and 45% in women are tobacco-related.
Condoms are effective 94% of the time.
Native Americans are significantly more likely to be hit crossing the streets than are people of other ethnicities.
People tend to be more persuasive when they look others directly in the eye and speak loudly and quickly.
Women make 75 cents to every dollar a man makes when they work the same job.
A surprising new study shows that eating egg whites can increase one’s life span.
People predict that it is very unlikely there will ever be another baseball player with a batting average over 400.
There is an 80% chance that in a room full of 30 people that at least two people will share the same birthday.
79.48% of all statistics are made up on the spot.

All of these claims are statistical in character. We suspect that some of them sound familiar; if not, we bet that you have heard other claims like them. Notice how diverse the examples are; they come from psychology, health, law, sports, business, etc. Indeed, data and datainterpretation show up in discourse from virtually every facet of contemporary life.

Statistics are often presented in an effort to add credibility to an argument or advice. We 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 consumers into decisions that might find cause to regret. For these reasons, learning about statistics is a long step towards taking “control of your life.” (It is not, of course, the only step needed for this purpose.) These next two chapters will help you learn statistical essentials. It will make you into an intelligent consumer of statistical claims.

You can take the first step right away. To be an intelligent consumer of statistics, your first reflex must be to question the statistics that you encounter. The British Prime Minister Benjamin Disraeli famously said, "There are three kinds of lies -- lies, damned lies, and statistics." This quote reminds us why it is so important to understand statistics. So, let us invite you to reform your statistical habits from now on. No longer will you blindly accept numbers or findings. Instead, you will begin to think about the numbers, their sources, and most importantly, the procedures used to generate them.

We have put the emphasis on defending ourselves against fraudulent claims wrapped up as statistics. Just as important as detecting the deceptive use of statistics is the appreciation of the proper use of statistics. You must also learn to recognize statistical evidence that supports a stated conclusion. When a research team is testing a new treatment for a disease, statistics allows them to conclude based on a relatively small trial that there is good evidence their drug is effective. Statistics allowed prosecutors in the 1950’s and 60’s to demonstrate racial bias existed in jury panels. Statistics are all around you, sometimes used well, sometimes not. We must learn how to distinguish the two cases.

8.1: Populations and Samples
Before we begin gathering and analyzing data we need to characterize the population we are studying. If we want to study the amount of money spent on textbooks by a typical first-year college student, our population might be all first-year students at your college. A sample is a smaller subset of the entire population, ideally one that is fairly representative of the whole population. We discuss sampling methods in greater detail in a later section.
8.2: 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. Categorical (qualitative) data are pieces of information that allow us to classify the objects under investigation into various categories. Quantitative data are responses that are numerical in nature and with which we can perform meaningful arithmetic calculations.
8.3: Sampling Methods
As we mentioned in a previous section, the first thing we should do before conducting a survey is to identify the population that we want to study. Suppose we are hired by a politician to determine the amount of support he has among the electorate. What population should we study? How do we choose an appropriate sample? There are many ways to sample a population, but there is one goal we need to keep in mind: we would like the sample to be representative of the population.
8.4: How To Mess Things Up
There are numbers 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 the bias introduced by only collecting data from those who volunteer to participate. This is not the only potential source of bias.
8.5: 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 some cases, these observations might be unsolicited. In other cases, the observations are solicited, like in a survey or a poll. 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.
8.6: Exercises
This page contains 22 exercise problems related to the material from Chapter 8.

Thumbnail: A visual representation of the sampling process. (CC BY-SA 4.0; Dan Kernler/modified by LibreTexts via Wikipedia)