Environmental monitoring: data trending using a frequency model

Environmental monitoring programs for the oversight of classified environments have used traditional statistical control charts to monitor trends in microbial recovery for classified environments. These methodologies work well for environments that yield measurable microbial recoveries. However, today successful increased control of microbial content yields numerous instances where microbial recovery in a sample is generally zero. As a result, traditional control chart methods cannot be used appropriately. Two methods to monitor the performance of a classified environment where microbial recovery is zero are presented. Both methods use the frequency between non-zero microbial recovery as an event. Therefore, the frequency of events is monitored rather than the microbial recovery count. Both methods are shown to be appropriate for use in the described instances.