Fig. 3. Four-cell diagram of dependence of water ecological state estimate on concentration of ammonium (mg/l).Numbers from 1 to 9 denote numbers of superimposed observations (sign "#" is used for numbers greater than 9). The horizontal dividing line is drawn at level F of state estimate, the position of vertical line is defined by ETL. The four-cell table at lower left corner contains the values of a, b, c, and d; the analogous table at the right contains the percents of these values to the total number of observation.