Histograms: Class Interval Width & Drawing the Histogram

How to Draw a Histogram:

1. Calculate the (ideal) number of classes
   • #classes = 1 + 3.3 log (n)
2. Calculate the class widths (using your result from step #1)
   • Class width = (largest observation – smallest observation)/(#classes)
3. Build a frequency distribution table
   • Begin your first class interval at the highest multiple of your class width that is just below your lowest observed value.
   • The second class interval starts at a value that equals the first  class interval PLUS the class width, etc.
   • Tally (count) the number of observations that fall within each class interval. Convert the tallies to frequencies. 
4. Draw the Histogram
   • Draw a horizontal line to show the classes.
     • Place evenly spaced marks along this line that correspond to the classes.
     • Label the marks so that the scale is clear and give a name to the horizontal axis.
   • Draw a vertical line just to the left of the lowest class.
     • Choose a scale for the vertical axis that will accommodate the class with the highest frequency.
     • Label the marks so that the scale is clear and give a name to the vertical axis.
   • Construct bars for each class. The height of each bar should correspond to the frequency of that class.