The EPA has promulgated regulations requiring certain stationary sources to show continual compliance with emission limits, by using a Continuous Emission Monitoring System (CEMS). In order to make reliable compliance determinations, the quality of the monitoring data is important. Two principal parameters in assessing and describing data quality are precision and accuracy. The paper discusses the long-term precision for five CEMS's installed at scrubber-controlled, coal-fired power plants. Precision estimates were made by utilizing available daily calibration drift data. Six to seven months of calibration drift data were analyzed for each pollutant (SO2, NOx) and diluent (O2, CO2) analyzer in the CEMS. Results show that most of the monitors exhibited significant bias and variance over the months involved. Monitoring data precision varied significantly from month to month and does not appear to be related to monitor type or manufacturer.