Use of confidence limits in the setting of On-Board Diagnostic thresholds

This Vehicles sold in the US and Europe have to be equipped with a Diagnostics, called On-Board Diagnostics (OBD), which monitor the performance of various elements of the emission control system. The driver is informed as to any failures by the use of a Check Engine Light on the dashboard of the vehicle and then should return the vehicle to the dealership for rectification. The vehicle manufacturer´s aim is to ensure that the Check Engine Light is only illuminated for legitimate failures. For the calibration of an On Board Diagnostics there needs to be sufficient separation between the response of a good sensor and a failed sensor, the setting of this threshold should be based upon a statistical model of the data so that the predicted failures rate can then be determined. By applying confidence limits to the models allows the engineer to understand how additional data points will effect the calculation of the failure threshold. This gives the engineer the ability to determine the tradeoff between the number of data points and a confidence in the estimated statistical model.