As the 20 observations were collected, a computer connected to the monitor calculated the average and the standard deviation of each sample. The rate at which the control limits are reset is determined by the caregiver. The caregiver is expected to examine the patient whenever the control limits are reset. Therefore, the caregiver may choose to reset the control limits when the average of one subgroup of observations is outside the control limits or the caregiver may prefer to recalculate the limits after the averages of several subgroups are beyond the control limits. The real-time patient control chart in this study is based on the Shewhart concept and includes an adjustment of the control limits for autocorrelation and data truncation.
Autocorrelated data are data that are anchored in the past.
For example, it is not possible to change from an overweight individual to an underweight individual over night. The change takes time because the weight of an individual is anchored to the past. The data collection rate was one observation per heartbeat and the subgroup size was based on 20 observations. A newborn may have a heart rate of 160 beats per minute. During one hour 480 subgroups of size 20 are generated.