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Data Literacy Project
 

DL
 SPSeries:
 10.19.11
 

Bar
 Graphs
 and
 Histograms:
 similarities
 and
 differences
 
A
 helpful
 description
 from
 Dr.
 Math
 [http://mathforum.org/dr.math/
 ]
 
  Questioner:
  “I'm
 confused
 about
 which
 graph
 has
 bars
 that
 touch
 on
 the
 sides,
 and
 which
 has
 bars
 that
 are
  separate.
 
 Let's
 say
 6
 girls
 love
 dogs,
 5
 love
 cats,
 and
 2
 love
 hamsters.
 
 A
 bar
 graph
 would
  represent
 this
 data
 better
 than
 a
 histogram,
 right?”
 
  Dr.
 Math
 replies:
  A
 histogram
 is
 a
 KIND
 of
 bar
 graph;
 so
 everything
 that
 a
 bar
 graph
 is,
 a
 histogram
 is
 too.
 
 But
  there
 are
 special
 things
 about
 a
 histogram
 that
 are
 not
 true
 of
 ordinary
 bar
 graphs,
 and
 these
  are
 the
 differences.
 
  One
 big
 difference
 is
 what
 they
 are
 used
 for.
 
 A
 bar
 graph
 can
 be
 used
 to
 compare
 ANY
 group
 of
  numbers,
 as
 in
 your
 example
 of
 the
 numbers
 who
 like
 different
 kinds
 of
 pets.
 
 That
 is
 very
 much
  like
 the
 sort
 of
 frequency
 distribution
 that
 histograms
 display,
 since
 you
 are
 graphing
  "frequencies";
 but
 the
 bars
 would
 represent
 categories,
 not
 numbers.
 
 
  A
 histogram
 is
 used
 ONLY
 when
 the
 bars
 will
 represent
 different
 numbers
 or
 intervals
 along
 an
  axis.
 
 For
 example,
 if
 6
 girls
 had
 pets
 weighing
 0
 -­‐
 3
 pounds,
 5
 had
 pets
 weighing
 3
 -­‐
 6
 pounds,
  and
 2
 had
 pets
 weighing
 6
 -­‐
 9
 pounds,
 you
 could
 make
 a
 histogram
 of
 that.
 
  As
 a
 result
 of
 this
 difference,
 we
 make
 the
 bars
 of
 a
 histogram
 touch
 -­‐-­‐that
 way,
 they
 illustrate
  the
 idea
 that
 they
 represent
 adjacent
 sets
 of
 numbers,
 and
 that
 together
 they
 cover
 an
 entire
  range
 of
 values.
 
 
 
 
  Discrete
 bar
 graphs
 represent
 separate
 entities,
 like
 your
 dogs,
 cats,
 and
 hamsters,
 and
  therefore
 are
 drawn
 separately,
 with
 space
 between.
 (Sometimes
 histograms
 are
 drawn
 with
  spaces,
 to
 indicate
 that
 they
 represent
 discrete
 numbers,
 that
 is,
 integers,
 rather
 than
 a
  continuous
 range
 of
 values.
 
 Also,
 sometimes
 at
 higher
 levels,
 the
 intervals
 can
 be
 different
  sizes,
 so
 the
 bars
 have
 different
 widths.
 
 Regular
 bar
 graphs
 always
 have
 the
 same
 width,
  because
 the
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