# DESCRIBING DATA

## DESCRIBING DATA

Once you have prepared a data file, you are ready to start analyzing the data. The first step in data analysis is describing the data. You look at the information you have gathered and summarize it in various ways. You count the number of people giving each of the possible responses. You describe the values by calculating averages and seeing how much the responses vary. You look at several characteristics together. How many men and how many women are satisfied with your new product? What are their average ages? You also identify values that appear to be unusual, such as ages in the one hundreds or incomes in the millions, and you check the original records to make sure that these values were picked up correctly. You do not want to waste time analyzing incorrect data.

## TESTING HYPOTHESES

To do this (and understand it), you have to learn something about statistical inference. Later chapters in this volume will show you how to test hypotheses and draw conclusions about populations based on samples. You will learn how to test whether you have sufficient evidence to believe that the differences or relationships you find in your sample are true for the whole population.

## DESCRIBING RELATIONSHIPS

You often want to determine what the relationship is between two variables. For example, what is the relationship between dollars spent on advertising and sales? How can you predict how many additional sales to expect if you increase your advertising budget by 25%? What is the relationship between the dosage of a drug and the reduction in blood pressure? How can you predict the effect on blood pressure if you cut the dose in half? You can study and model the relationship between pairs of variables in many different ways. You can compute indexes that estimate the strength of the relationship. You can build a model that allows you to predict values of one variable based on the values of another. That is what the last part of the book is about.

You must state your ideas clearly if you plan to evaluate them. This advice applies to any kind of work but especially to research design and statistical analysis. Before you begin working on design and analysis, you need to have a clearly defined topic to investigate.