# 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.