The p chart monitors the fraction or proportion of defective or nonconforming units. Each point on the chart represents the sample ratio of defective units to the total number inspected within each subgroup or time interval. The solid centerline is the average of the sample proportions. The two dashed lines are the upper control limit (UCL) and the lower control limit (LCL). Note that the value of the UCL is variable because the sample size varied by collection period. The value of the LCL is near 0.0, so it's not visible because it's flush with the horizontal axis. Control limits reflect the expected amount of variation in the number of defective units when only common causes of variation are present. This chart looks good because all the points are within the control limits and there are no obvious patterns.