Читать книгу Improving Health Care Quality - Cecilia Fernanda Martinez - Страница 25

In practice, process changes are most effective right after they are implemented when awareness is high, but over time, these changes may not be sustained. Control charts are a key tool for monitoring quality improvements to be sure that the desired effect is maintained over time. They track key process variables and alert the user when something has changed in the process performance.

An important feature of control charts is that they display the region corresponding to the expected variability (common cause variation) of a process indicator when it is operating normally (i.e. in control). Observations lying outside of this region alert the user that the process has changed and action should be taken to understand what has changed and deploy any needed corrective actions. There are different types of control charts that depend on the measurement level of the process variable. “Variables” charts are for those variables measured on a continuous scale. “Attribute” charts apply to count measurements, such as number of errors per insurance claim. Figure 1.4 shows some of the commonly applied attribute and variables charts.

Figure 1.4 Types of control charts.

Attributes are counts, classified as either defectives or defects. A defective is an item that does not meet the requirements, while defects are the number of nonconformances per item. For example, consider the process of hospital bills being audited periodically. If a bill contains an error, it would be considered defective, and the count of all defective bills during the audit period would be appropriately monitored by P‐ or NP‐charts, depending on whether the number of bills audited in each period is variable or fixed, respectively. In contrast, if the auditors count the number of errors on each bill, then U‐ and C‐charts are applicable, again depending on whether the number of bills audited in each period is variable or fixed, respectively.

A process variable that is continuous is typically monitored using two charts, one to track the process average and one to track the variation. The I–MR chart combination (in JMP referred to as IR) allows a process to be monitored when there is only one observation per unit time. For example, an emergency department monitoring the number of hours they are not accepting incoming ambulances (ambulance diversion) would employ I‐ and MR‐charts. X‐bar and either an S‐ or R‐chart are applicable when monitoring a process where there are multiple observations per time period, such as the waiting time until a patient completes the intake process in an emergency department. The case “Monitoring Ambulance Diversion Hours” explains the construction and use of I‐ and MR‐control charts. Chapter 11 illustrates several types of control charts. Additional information on control charts can be found in Montgomery (2012).