Introduction

Why do X-ray astronomers need statistics? Wall and Jenkins (2003) give a good description of scientific analysis and answer this question. Statistics are used to make decisions in science, evaluate observations, models, formulate questions and proceed forward with investigations. Statistics are needed at every step of scientific analysis. A statistic is a quantity that summarizes the data (mean, averages etc.) and astronomers cannot avoid statistics.

Here is a question asked by an X-ray astronomy school student:

What does “a good fit” or “a decent fit” mean, and what constitute “low counts” data? These expressions carry a definite meaning, but taken out of context are not precise enough. Is the question whether the total number of counts in the spectrum is “low” or whether the number of counts per resolution element is “low”? For example, a total number of counts such as 2000 could be viewed as “high,” but it might be considered “low” if the total number of counts is divided by the number of resolution elements (for example there are 1024 independent detector channels in a Chandra ACIS spectrum).