Chapter 10 STATS.docx

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30 Mar 2012
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Chapter 10
Graphs, Good and Bad
What makes a clear table?
It is clearly labelled so that we can see the subject of the data at once
Labels within the table identify the variables and state the units in which they are measured
The source of the data appears at the foot of the table
Pg 194
One of the tables starts with the counts of the people
Rates are often clearer than counts
The last two columns of the table present the distribution of the variable
The distribution of the variable tells us what values it takes and how often it takes these values
Roundoff errors (the rounded entries don’t quite add to the total)
Pie Charts and bar graphs
Pie chart show how a whole is divided into parts (the circle represents the whole, wedges within
the circle represent all parts, with the angle spanned by each wedge in proportion to the size of
that part)
Pie charts force us to see that the parts do make a whole
A pie chart is not a good way to compare the sizes of the various parts of the whole
Bar graph (the height of each bar shows the percent/unit)
The bar graph is also easier to draw than the pie chart unless a computer is doing the drawing for
you
The bar graph is also easier to draw than the pie chart unless a computer is doing the drawing for
you
It is helpful to distinguish between variables whose values have a meaningful numerical scale and
variables such as sex, occupation or level of education
Pie charts and bar graphs are most useful for the second kind of variable
A categorical variable places an individual into one of several groups or categories
A quantitative variable takes numerical values for which arithmetic operations such as adding
and average make sense
A pie chart can only compare parts of a while
Bar graphs can compare quantities that are not parts of a whole
Beware of pictogram
Bar graphs compare several quantities by contrasting the heights of bars that represent the
quantities
Our eyes react to the area of the bars as well as to their height
When all bars have the same width, the area (the width x height) varies in proportion to the height
and our eyes receive the right impression
Ex. Pens graph
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