30 Mar 2012

School

Department

Course

Professor

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