Forfatter av avsnitt: Danielle J. Navarro and David R. Foxcroft

Deskriptivstatistikk

Any time that you get a new data set to look at one of the first tasks that you have to do is find ways of summarising the data in a compact, easily understood fashion. This is what descriptive statistics (as opposed to inferential statistics) is all about. In fact, to many people the term “statistics” is synonymous with descriptive statistics. It is this topic that we will consider in this chapter, but before going into any details, let us take a moment to get a sense of why we need descriptive statistics. To do this, let us open the aflsmall_margins data set and see what variables are stored in the file (Fig. 20).

Variabler lagret i ``aflsmall margins`` — Fig. 20 Skjermbilde av jamovi som viser variablene i datasettet `aflsmall_margins`

In fact, there is just one variable here, afl.margins. We will focus a bit on this variable in this chapter, so I should better tell you what it is. Unlike most of the data sets in this book, this is actually real data, relating to the Australian Football League (AFL).[1] The afl.margins variable contains the winning margin (number of points) for all 176 home and away games played during the 2010 season.

This output does not make it easy to get a sense of what the data are actually saying. Just “looking at the data” is not a terribly effective way of understanding data. In order to get some idea about what the data are actually saying we need to calculate some descriptive statistics (this chapter) and draw some nice pictures (chapter Lage diagram). Since the descriptive statistics are the easier of the two topics I will start with those, but nevertheless I will show you a histogram of the afl.margins variable since it should help you get a sense of what the data we are trying to describe actually look like, see Fig. 21. We will talk a lot more about how to draw histograms. For now, it is enough to look at the histogram and note that it provides a fairly interpretable representation of the afl.margins variable.