Ring Diagrams Overview

Ring Diagram Legend

GOOD - Green Hexagon

Measurement is within the interquartile range (IQR), defined as the 25th to 75th percentile or middle half (50%) of the measurements. Examples: sulfate, ammonium, sodium, and potassium.

SATISFACTORY - Blue Trapezoid

Measurement is within the range defined by the median + IQR/1.349. The ratio, IQR/1.349, is the non-parametric estimate of the standard deviation, sometimes called the pseudo-standard deviation. Examples: nitrate, chloride, and calcium.


Measurement is outside the range defined by the median + IQR/1.349. Examples: pH, conductivity, and magnesium.


Measurement is below the detection limit of the laboratory’s analytical method. Example: fluoride.


  1. An outward pointing trapezoid or triangle indicates the measurement is high. An inward pointing trapezoid or triangle indicates the measurement is low.
  2. A hexagon or trapezoid with cross-hatching (for example: potassium) indicates a measurement that meets the GOOD or SATISFACTORY definition but fails the measurement Data Quality Objective (DQO), defined in the Manual for the GAW Precipitation Chemistry Programme and listed in Table 1, below.
  3. A straight line without a symbol indicates no measurement was reported. For example: acidity.


Table 1. Precipitation Chemistry
Data Quality Objectives

Measurement     DQO
pH + 0.07 units
conductivity + 7%
acidity + 25%
sulfate + 7%
nitrate + 7%
ammonium + 7%
fluoride none
chloride + 10%
sodium + 10%
potassium + 20%
calcium + 15%
magnesium + 10%