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Published online by Cambridge University Press: 05 June 2012
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:
I wanted to know how many counts would be needed to get a good fit for a CIE plasma model with every parameter (save redshift) free. I was once told that it took 500–1000 counts to get a decent fit, but I couldn't remember if this assumed that metallicity is fixed. Can someone get a good fit for metallicity with so few counts?
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).
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