Exposure assessment in investigations of waterborne illness: a quantitative estimate of measurement error

Table 1 Performance of two surrogate measures for home water source (a) residential postal code-based urban/rural status and (b) GIS-assigned home water source, using resident-reported home water source as the gold standard

Home Water Source	Self-reported Municipal Water Source	Self-reported Private Water Source	Total
Postal-Code Based Urban/Rural Status
Urban designation	1318	21	1339
Rural designation	48	95	143
Total	1366	116	1482
Sensitivity	0.96
Specificity	0.82
Positive Predictive Value^†	0.98
Negative Predictive Value*	0.66
GIS-mapping to Water System
Municipal system assignment	1310	27	1337
Private system assignment	56	88	144
Total	1366	115	1481
Sensitivity	0.96
Specificity	0.77
Positive Predictive Value^§	0.98
Negative Predictive Value^Φ	0.61

^†The proportion of urban-designated residences that truly had municipal water systems
* The proportion of rural-designated residences that truly had private water systems
^§ The proportion of residences assigned to municipal water systems that truly had municipal water systems
^ΦThe proportion of residences assigned to private water systems that truly had private water systems