FICCI CE Control charts - P charts The p charts are used to control the overall number of defective units in a process. The proportion of non conforming items are plotted on this chart which gives us the graphical display of variability of the data.
FICCI CE Construction of P charts Number of defective units Data points P= Number of units in the sample Total defective units Process Average P= = 0.053 Total units observed Upper control Limit ( UCL ) = P+3 P ( 1-P ) = 0.0926 n = P-3 P ( 1-P ) Lower control Limit =0.1508 n n=Average sample size= 305* * Since the data size do not vary much around the mean value, P chart is simplified by taking the average sample size to calculate he control limits.
FICCI CE Construction of P charts The construction of control charts is very simple. Simply select the range of data points ( “proportion” column in the excel sheet ) and click the graph button. Select “Line graph” and the p chart is automatically calculated. The average, UCL and LCL can be manually displayed by drawing three additional lines on the graph. Please refer the next slide for the graph.
FICCI CE P chart 0.1 0.09 UCL 0.0926 0.08 0.07Proportion 0.06 CL 0.05 0.053 0.04 0.03 0.02 LCL 0.01 0.015 0 5 Days 15 25
FICCI CE NP charts The NP chart is used for plotting number of defectives instead of proportion defectives. This is another version of P chart and its applicability depends upon the interest of the end user.