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MA181 INTRODUCTION TO STATISTICAL MODELLING

GRAPHICAL REPRESENTATION OF DATA

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\item[Dotplot]
For all but the smallest samples, some form of graphical
representation of the data is often useful in giving guidance as
to how they should be analysed, or perhaps how they should nut.
With less than 30 observations of a continuous random variable,
plotting their values on a line probably provides the most
valuable picture for, while with such a small sample one would not
expect to be able to determine with any great accuracy the
distribution from which the data have arisen, such a simple
diagram, known as a \textit{dotplot}, may be sufficient to show
that, for instance, the assumption of a symmetric distribution
would be unwise. As an example, consider these 20 measurements
made of the lifetime of an electric light bulb (in hours):

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975, 992, 1063, 1197, 1041, 1088, 960, 1039, 1031, 983

1147, 1006, 1070, 1013, 1118, 1021, 1051, 966, 998, 1302.

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Plotting these values on a line produces the following diagram,
which shows the data clustering on the left but more spread out on
the right, suggesting that the distribution of the lifetime of the
bulb is probably not symmetric, let alone bell-shaped, but skewed
to the right, that is with a short left-hand tail and a long
right-hand one.

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When data have been rounded, it is quite possible for several
observation to take the same value. In this case, equal points on
the dotplot may be stacked, one above the other.

With 30 observations or more, it becomes feasible to draw a two
dimensional picture of the sample distribution. The traditional
diagrams used foe this purpose are the bar chart and the
histogram, depending on whether the observations are of a discrete
or a continuous random variable. In fact, the bar chart can prove
quite successful even for small samples. More recently, a number
of alternative diagrams have been suggested, due largely to the
influence of the American statistician John Tukey. Two of these
are mentioned later.

\item[Bar chart]
Data from a discrete random variable can be represented by a
\textit{bar chart}, in which the possible values of the variable
are marked along the horizontal axis (over the range of values for
which observations occur) and a line, or bar, is drawn at each,
parallel to the vertical axis, with length proportional to the
frequency of the observations at that value. This is illustrated
by the following diagram representing the number of occurrences of
the word \lq\lq the '' in blocks of 40 words from Milton's
\textit{Paradise Lost}. The values 0-6 were observed, and these
are shown marked with their respective bars. The height of any
bar, and so the frequency of any observed value, can be read from
the scale on the left. The discreteness of the random variable
measured is clearly indicated by the separation of the bars.

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\item[Histogram]
A continuous random variable takes values over an interval (or the
whole of) the real line, and an adequate representation of such a
variable needs to make this clear, its picture should cover the
$x$-axis. That being said, it is important to realise that any
continuous variable can be measured only to a certain accuracy, in
other words the observations made are, strictly speaking,
discrete. Nonetheless, it is usual to create a diagram that
displays the continuous nature of the random variable measured
rather than the discreteness of the observations.

The \textit{histogram}, a diagram that has been used for many
generations, consists of a number of rectangles based on the
$x$-axis, each with an \textit{area} proportional to the frequency
of the observations lying in the interval it covers. It is usual
for most, if not all, of these intervals, known as \textit{class
intervals}, to have the same width except where the data becomes
sparse, in which regions wider intervals are sometimes used. The
choice of interval width, and so the number of rectangles drawn,
requires some care, too many rectangles and the outline of the
distribution of the data is dominated by random fluctuations, too
few and important features in the outline are smoothed out. Except
with a very large data set, a histogram should contain 5-20
rectangles, with the exact number chosen being guided by the
problems just mentioned and the desirability of placing
\textit{class boundaries}, the boundaries between class intervals,
at convenient points. These should not be at values taken by any
of the observations, otherwise problems arise in determining into
which intervals those particular observations should be placed; it
is usually best to set them midway between observations, or
possible values of observations.

Consider the histogram drawn below of the percentage of butterfat
from 120 three-year-old Ayrshire cows selected at random from a
Canadian stock record book. The observations are of a continuous
random variable, each one being given correct to one decimal
place, so we assume that a reading like 3.7\% is a rounded value
of measurement lying in the interval 3.65-3.75\%. When drawing a
histogram of data, we could choose class intervals of 0.1\%, which
would result in 16 rectangles, or of width 0.2\%, resulting in
only eight rectangles. The former leads to raggedness in the
extremes of the picture, while the latter seems to be throwing
away just a little too much detail. A compromise is possible, in
which intervals of width 0.1\% are used in the center of the
diagram and of width 0.2\% in the tails. This is how the graph has
been drawn, with 11 intervals and an outline that probably gives a
good approximation to that of the distribution of the random
variable measured. It is worth noting the following features:

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\item[(i)]
The area, and not the height, of each rectangle is proportional to
the number of observations contained in the class interval at its
base. Only if all the intervals are of the same width are the
heights of the rectangles proportional to the frequencies. As a
consequence, it is not possible here to mark the frequencies on a
vertical scale along side the histogram. Instead, they are given
inside the rectangles toward the bottom.

\item[(ii)]
The class boundaries have all been set midway between pairs of
possible values, i.e. at 3.45\%, 3.65\%, 3.85\%, 3.95\%, etc. In
this way, no observation arises with the possibility of being
placed in more than one class interval.

\item[(iii)]
There is more than one way of marking relevant positions on the
$x$-axis. One of these is to mark the class boundaries. However,
these often involve more decimal digits than the data themselves,
so giving a cumbersome appearance. Perhaps the simplest is to mark
just the midpoints, or \textit{class marks}, of the intervals and
to calculate anything else as and when required. This is the
method adopted in the figure.

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\item[Stem-and-leaf plot]
The \textit{stem-and-leaf plot} can be drawn for either a discrete
or a continuous random variable. It looks somewhat like a barchart
or a histogram on its side, except that the bars or rectangles are
constructed from the very digits of data themselves. In this
respect, for a sample from a continuous random variable, the
diagram is strictly a representation of the observed rounded data.

The stem of a stem-and-leaf plot consists of the most significant
digits of the observations, while the leaves consist of the least
significant, with a vertical line separating the two parts. In
some cases, this line will represent the decimal point in the
data.

The stem-and-leaf plot below shows the total scores of the 60
golfers who played all four rounds in the 1985 British Open golf
tournament. The totals range from 282 to 301, the first two digits
forming the stem and the last digit the leaf. The column on the
left gives the cumulative counts of the data from both ends except
that the number in brackets indicates the row in which the sample
median lies and gives the count just for that row.

\begin{tabular}{rrr|l}
2&&28&23\\ 10&&28&44444555\\ 19&&28&666667777\\
(13)&&28&8888899999999\\ 28&&28&001111\\ 22&&29&22222333\\
14&&29&444444455\\ 5&&29&77\\ 3&&29&8\\ 2&&30&01
\end{tabular}

\item[Boxplot]
A \textit{boxplot} (or \textit{box-and-whisker plot} is a graph
that illustrates the location and spread of a set of observations,
highlighting any that are extreme in their values. The central box
extends from the lower quartile, $Q_1$, of the data to the upper
quartile, $Q_3$, with the median being marked by an internal line
across the box. The distance $H=Q_3-Q_1$ is known as the
interquartile range. Whiskers are drawn from each end of the box
extending as far as $1,5H$, or as far as the furthest observation
within that range. Any observations lying even further out, known
as outliers, are drawn as separate dots.

The boxplot below illustrates the butterfat data described above.
The median is at 4,15, $Q_1=4,00$ and $Q_2=4,35$, so that
$H=0.35$. The lower whisker, which could extend down to 3.475,
needs go no lower than the smallest observation, 3.5, while the
upper whisker, which could extend to 4.875, stop at the
observation 4.8. Consequently, three points (4.9,4.9 and 5.0) are
outliers in the right extremity of the graph.

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