EPA's 25th Percentile Approach

This approach involves assembling all available data (in this case the long term phosphorus values for the 82 reservoirs) and ordering the values from lowest to highest. The 25th percentile of the data (phosphorus value of the reservoir that is one quarter the way up the list) is selected as the criterion. The 25th percentile of the phosphorus data for the 82 reservoirs in this analysis would be 34 ug/L, which would lead to 62 reservoirs being listed as impaired.

The 25th percentile of data is easily viewed by looking at a box plot (like you‘d see in the LMVP Data Report). The values from the bottom line to the bottom of the box represent the 25th percentile.

For more help with box plots visit our Understanding the Data page.