Consider again the quiz scores we looked at when we discussed dotplots:Because the data are integral and range from 15 to 50, reasonable intervals might be of size 10 or of
size 5. The graphs below show what happens with the two choices:
Typically, the interval with midpoint 15 would have class boundaries 12.5 ≤ x <17.5; the interval
with midpoint 20 would have class boundaries 17.5 ≤ x <22.5, etc.
There are no hard-and-fast rules for how wide to make the bars (called “class intervals”). You should
use computer or calculator software to help you find a picture to which your eye reacts. In this case,
intervals of size 5 give us a better sense of the data. Note that some computer programs will label the
boundaries of each interval rather than the midpoint.