182 Statistics BRITANNICA STUDENT ENCYCLOPEDIA
large groups of people—for example, the
reading level of students, the opinions of
voters, or the average weight of a city’s
residents.
Collecting Data
The people who gather statistics are
called statisticians. They collect pieces
of information, or data, in three basic
ways. They refer to trustworthy sources,
such as newspapers and reports from
reliable organizations. They perform
scientific experiments. They also
conduct surveys, or ask people
questions.
Sometimes statisticians cannot gather
data about every member of a group,
often because the group is too large. In
such cases they study only part of the
group, called a sample. A sample must
accurately represent the larger
population. For example, if women
make up 60 percent of the population,
then 60 percent of the sample must be
women.
Summarizing Data
Once statisticians have collected their
information, they summarize it. They
often put the data in graphs or charts,
which are easier to read than long lists of
data.
Statisticians also summarize data by calculating
numbers called averages. There
are three kinds of averages: mean, mode,
and median.
The mean is the sum of a set of numbers
divided by the amount of numbers in
the set. If 100 people weigh a total of
1,500 pounds, then the mean weight is
1,500 pounds divided by 100, or 150
pounds. The mean is the most common
kind of average.
The mode is simply the number that
occurs most often in a sample. If the
most common weight in a sample of
people is 139 pounds, then the mode is
139. Modes also can describe information
not made up of numbers—for
example, letter grades.
The median is the number right in the
middle of the sample. If the sample
weights are 125, 130, 145, 150, and 160
pounds, then the median weight is 145
pounds. Medians are helpful when there
are some unusually high or low numbers
in the sample.
Using Statistics
Once the data are summarized, people
interpret them, or decide what they
mean. Newspaper articles, books, and
Every sport has its own set of statistics for its
athletes. Fans often use those statistics to
compare the athletes.
BRITANNICA STUDENT ENCYCLOPEDIA Statistics 183
political speeches often include statistics.
People use them to support their opinions.
Because statistics are represented by
charts, graphs, and numbers, people
tend to believe them. However, people
can use statistics to twist the truth. It is
important to know whether the statisticians
collected the data carefully and
summarized them accurately.
#More to explore
Graph • Mathematics • Numbers and
Number Systems
Statue of Liberty
#see Liberty, Statue of.
Steel
Steel is a hard metal that people use to
make many different products. Car bodies,
bridges, building frames, and containers
are just a few of the things made
from steel.
Steel is basically iron mixed with a little
carbon. But steel is better than iron in
many ways. Steel is stronger but lighter
than iron. Steel also lasts longer than
iron.
To make steel, workers heat iron until it
melts. Then they add carbon to the liquid
iron. They may also add other materials
to make different types of steel. For
example, steel with added chromium is
called stainless steel. It will not rust or
stain. Finally, workers use machines to
cool the liquid steel into bars, slabs, or
long sheets. The cooled, hardened steel
may then be made into products.
People made the first steel as early as
1400 BC. Ancient people heated iron
inside a pile of burning charcoal. This
added carbon to the iron. The result was
not the same as modern steel, but it was
stronger than regular iron.
In the 1800s people began to use huge
ovens to make large amounts of steel. In
the late 1800s builders used steel to
make some of the first skyscrapers. They
also used steel to make large, modern
bridges. In the 1900s new processes
made steelmaking easier. Today steel
remains one of the most popular building
materials in the world.
#More to explore
Iron • Metal • Skyscraper
Stegosaurus
The name Stegosaurus means “covered
lizard” or “roof lizard.” The dinosaur
called Stegosaurus was given this name
Long sheets of steel are stored on rolls in a
factory called a steel mill.
184 Statue of Liberty BRITANNICA STUDENT ENCYCLOPEDIA
because it had a row of enormous bony
plates that ran along its back. Stegosaurus
is the most familiar and the largest of
the stegosaurs, or plated dinosaurs.
When and Where Stegosaurus
Lived
Stegosaurus lived about 159 to 144 million
years ago. Fossils, or remains, of
Stegosaurus have been found in Colorado,
Wyoming, Utah, and Oklahoma.
Stegosaurus lived in wooded areas with
low-growing types of plants.
Physical Features
Stegosaurus was a massive dinosaur that
lumbered along on all four legs. It usually
grew to a length of about 21 feet
(6.5 meters). Stegosaurus measured about
10 feet (3 meters) tall at the hips and
weighed about 2 tons. The huge plates
that ran along its back from its neck to
its tail stood in two staggered rows down
either side of the backbone. Some of the
plates were more than 2 feet (60 centimeters)
tall. Stegosaurus also had two to
