Page 120 - Statistics for Dummies

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Part II: Number-Crunching Basics
Making a histogram
A histogram is a special graph applied to data broken down into numerically
ordered groups; for example, age groups such as 10–20, 21–30, 31–40, and
so on. The bars connect to each other in a histogram — as opposed to a bar
graph (Chapter 6) for categorical data, where the bars represent categories
that don’t have a particular order, and are separated. The height of each bar
of a histogram represents either the number of individuals (called the fre-
quency) in each group or the percentage of individuals (the relative frequency)
in each group. Each individual in the data set falls into exactly one bar.
You can make a histogram from any numerical data set; however, you can’t
determine the actual values of the data set from a histogram because all you
know is which group each data value falls into.
An award winning example
Here’s an example of how to create a histogram for all you movie lovers out
there (especially those who love old movies). The Academy Awards started in
1928, and one of the most popular categories for this award is Best Actress in
a Motion Picture. Table 7-1 shows the winners of the first eight Best Actress
Oscars, the years they won (1928–1935), their ages at the time of winning their
awards, and the movies they were in. From the table you see the ages range
from 22 to 62 — much wider than you may have thought it would be.
Table 7-1 Ages of Best Actress Oscar Award
Winners 1928–1935
Year Winner Age Movie
1928 Laura Gainor 22 Sunrise
1929 Mary Pickford 37 Coquette
1930 Norma Shearer 30 The Divorcee
1931 Marie Dressler 62 Min and Bill
1932 Helen Hayes 32 The Sin of Madelon Claudet
1933 Katharine Hepburn 26 Morning Glory
1934 Collette Colbert 31 It Happened One Night
1935 Bette Davis 27 Dangerous

To find out more about the ages of Best Actresses, I expanded my data set to
the period 1928–2009. The age variable for this data set is numerical, so you
can graph it using a histogram. From there you can answer questions like:
What do the ages of these actresses look like? Are they mostly young, old,
in between? Are their ages all spread out, or are they similar? Are most of

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