Calculating or Determining the 90th Percentile
during Initial, Follow-up, Routine, and
Reduced Monitoring
If you collect 5 samples, calculate your 90th percentile like this:
• Rank your samples in order of concentration (mg/L), starting from lowest to highest.
• Find the average of the two highest results by adding them together and dividing by 2.
• The resultant number is recorded for the 90th percentile.
Sample Site #
Sample results
1
0.001
2
0.001
3
0.006
4
0.008
5
0.014
0.008
+
0.014
0.022 . . . . . . .
.
Average
divide 0.022 by 2 = 0.011
(This is an example. Insert your own sample results.)
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ** * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
*
Calculating the 90th Percentile for 6 samples or more:
• Rank your samples in order of concentration starting from lowest to highest.
• Take the total number of samples collected and multiply it by 0.90. The resultant number
will tell you which sample to record.
•
If the number is not a whole number, you need to round to the nearest whole
number.
•
If the number is exactly in the middle of two whole numbers, you round to the
nearest even number.
•
i.e. 12.5 would be rounded to 12, and 13.5 would be rounded to 14.
Example: If you collect 10 samples, determine your 90th percentile like this:
10 x 0.90 = 9
Sample Site #
Sample results
1
0.001
2
0.001
3
0.001
4
0.001
5
0.001
6
0.004
7
0.005
8
0.006
9
0.008
10
0.010
(This is an example. Insert your own sample results.)
This sample should be recorded as
the 90th percentile result on Form
141A.
This is the number to
record on Form 141A.
(Use this format if your laboratory does not provide a 90th
percentile summary for you)
Public Water Supply ID No. __________________
Name of Public Water Supply: ____________________________________________________________
Results of Monitoring: