Cumulative Frequency
This is the running total of the frequencies. On a graph, it can be
represented by a cumulative frequency polygon, where straight lines join up the
points, or a cumulative frequency curve.
Example
Frequency: 
Cumulative Frequency: 

4 
4 

6 
10 
(4 + 6) 
3 
13 
(4 + 6 + 3) 
2 
15 
(4 + 6 + 3 + 2) 
6 
21 
(4 + 6 + 3 + 2 + 6) 
4 
25 
(4 + 6 + 3 + 2 + 6 + 4) 
The Median Value
The median of a group of numbers is the number in the middle, when the
numbers are in order of magnitude. For example, if the set of numbers is 4, 1,
6, 2, 6, 7, 8, the median is 6: 1, 2, 4, 6, 6, 7, 8 (6 is the
middle value when the numbers are in order) If you have n numbers in a group,
the median is the (n + 1)/2 th value. For example, there are 7 numbers in the
example above, so replace n by 7 and the median is the (7 + 1)/2 th value = 4th
value. The 4th value is 6.
When dealing with a cumulative frequency
curve, "n" is the cumulative frequency (25 in the above example). Therefore the
median would be the 13th value. To find this, on the cumulative frequency curve,
find 13 on the yaxis (which should be labelled cumulative frequency). The
corresponding 'x' value is an estimation of the median.
Quartiles
If we divide a cumulative frequency curve into quarters, the value at the
lower quarter is referred to as the lower quartile, the value at the middle
gives the median and the value at the upper quarter is the upper quartile. A
set of numbers may be as follows: 8, 14, 15, 16, 17, 18, 19, 50. The mean of
these numbers is 19.625 . However, the extremes in this set (8 and 50) distort
this value. The interquartile range is a method of measuring the spread of the
middle 50% of the values and is useful since it ignore the extreme values.
The lower quartile is (n+1)/4 th value (n is the cumulative
frequency, i.e. 157 in this case) and the upper quartile is the 3(n+1)/4 the
value. The difference between these two is the interquartile range (IQR). In
the above example, the upper quartile is the 118.5th value and the lower
quartile is the 39.5th value. If we draw a cumulative frequency curve, we see
that the lower quartile, therefore, is about 17 and the upper quartile is about
37. Therefore the IQR is 20 (bear in mind that this is a rough sketch  if you
plot the values on graph paper you will get a more accurate value).
Copyright © Matthew Pinkney 2003
