Estimated interquartile ranges (IQRs) within and between zones for individual THMs (μg/l) in the north west of England
| THM | Year | IQR between zones | Average IQR within zones weighted by number of samples/zone | Median IQR within zones | Percentage of IQRs within zones that are less than the IQRs between zone |
| Chloroform | 1992 | 27.4 | 24.9 | 23.5 | 74.7 |
| 1993 | 30.9 | 27.3 | 23.2 | 78.8 | |
| 1994 | 32.2 | 21.9 | 19.6 | 92.3 | |
| 1995 | 28.1 | 13.7 | 13.7 | 96.4 | |
| 1996 | 26.8 | 16.7 | 15.2 | 93.8 | |
| BDCM | 1992 | 4.5 | 4.5 | 3.9 | 67.1 |
| 1993 | 4.3 | 4.4 | 3.8 | 66.0 | |
| 1994 | 4.3 | 3.6 | 3.2 | 78.9 | |
| 1995 | 4.6 | 3.1 | 2.84 | 94.1 | |
| 1996 | 4.6 | 3.0 | 2.7 | 90.5 | |
| DBCM | 1992 | 0.7 | 1.1 | 0.6 | 57.4 |
| 1993 | 0.7 | 1.0 | 0.6 | 65.7 | |
| 1994 | 1.0 | 1.2 | 0.8 | 61.1 | |
| 1995 | 1.1 | 1.2 | 0.8 | 58.2 | |
| 1996 | 1.22 | 1.3 | 0.7 | 61.5 | |