TITLE: Simple question about histogram
QUESTION [3 upvotes]: Wikipedia article about histograms says following:

A histogram is a representation of tabulated frequencies, shown as
  adjacent rectangles, erected over discrete intervals (bins), with an
  area equal to the frequency of the observations in the interval. The
  height of a rectangle is also equal to the frequency density of the
  interval, i.e., the frequency divided by the width of the interval.

and yet, its own example about "heights of Black Cherry trees" has heights equal to frequency of observations. So when is the height frequency density and when is it frequency observations?

REPLY [0 votes]: The area of a rectangle in a histogram is equal to the frequency of the observations in the interval represented by the rectangle.
On the other hand, the height of a rectangle in a histogram is equal to the frequency density of the interval. This is calculated as the area of the rectangle (frequency) divided by the width of the interval. To better understand this concept, let us consider two histogram rectangles with equal interval (e.g., age between 50 and 60, and between 60 to 70) and different heights. The higher rectangle indicates that, despite equal intervals, it contains more observations than the other. Thus, it is more "dense".
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