BOX and WHISKER PLOTS

[greese]

When drawing a box and whisker plot of a frequency table, do we use the upper class boundary or the midpoint of the class?

For example:

length: 0-9 10-19 20-29 30-39
Number: 28 34 17 6

Do we input this data as 28- 4.5's or 28- 9.5s?

When finding the mean of this data we use the midpoint of each class correct? Likewise for median? What about mode? Do we just say the mode is the range 10-19?

thanks for your help

 

[Loren]

If I understand your question correctly, the assumption is made that the 28 cases that lie within the boundaries of 0 to 9 are roughly evenly distributed within that class. So, you use the midpoints.

 

[greese]

ok -- but am i correct that for a cumulative frequency plot (ogive) you would use the upper class boundary?

in which case the median and quartiles would be different values for the box and whisker compared to the ogive if the same question is done