Boxplots
Typically Boxplot presents five sample statistics - the minimum,
the lower quartile, the median,
the upper quartile and the maximum - in a visual display. It
is a rectangle which encloses the middle half of the sample, with an end at
each quartile. The length of the box is thus the interquartile
range of the sample. The other dimension of the box does not represent
anything in particular. A line is drawn across the box at the sample median.
Whiskers sprout from the two ends of the box until they reach the sample
maximum and minimum. The crossbar at the far end of each whisker is optional
and its length signifies nothing. The following diagram shows a dotplot of a
sample of 20 observations together with a boxplot of the same data.
Although boxplots can be drawn in any
orientation, but SPSS produce them vertically by default. The length of the box
becomes its height. The width across the page signifies nothing.
The box length gives an indication of the sample
variability and the line across the box shows where the sample is centered. The
position of the box in its whiskers and the position of the line in the box
also tells us whether the sample is symmetric or skewed. For a symmetric
distribution, long whiskers, relative to the box length, can betray a heavy tailed population and
short whiskers, a short tailed population. So, provided the number of points in
the sample is not too small, the boxplot also gives us some idea of the
"shape" of the sample, and by implication, the shape of the
population from which it was drawn. This is all important when considering
appropriate analyses of the data. Boxplots are not as adept as histograms in providing
information about the shape of a distribution
Rohit Jayant
14160
Operation batch
Group D
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