What is the difference between p chart and np chart

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Views Total views. Actions Shares. No notes for slide. Control charts p np c u 1. Attribute are used: 1. These control limits are exact solutions of the equation governing the binomial distribution with the assumption that the probability P of obtaining a point beyond the control limits is less than or equal to 0.

The limits given in the table are for np charts for various values of p and n. To obtain the limits for a p chart or convert np to p, use the following relationships:. To understand how to use the table and how it was developed, consider the following example. Suppose you are sampling 10 items such as invoices or expense accounts on a regular basis. The average fraction defective has been determined to be 0.

The table gives the control limits for the np chart. The LCLp is actually The control limits in the table were obtained from the equation governing the binomial distribution. In Microsoft Excel, you can use the function "Binomdist" to determine this. The calculation results are summarized below. The probability of the sample containing 0 defective items is out of 1, The probability of the sample containing 0 or 1 defective item is out of 1, The probability of the sample containing 0, 1, or 2 defective items is out of 1, The control limits in the table are determined so that the probability of obtaining a point beyond the control limits is less than or equal to 0.

For this case, the probability becomes less than 0. Thus, the upper control limit for this example is 3. It has values of n to 50 and pbar to 0. The control limit equations for the np chart give the following results:. If this is not true, the binomial distribution which governs the p and np control charts is not symmetrical. This is called the small sample case for the p and np control charts. Thanks so much for reading our publication.

We hope you find it informative and useful. Happy charting and may the data always support your position. In np-chart, number of defectives is plots on the y-axis and the number of samples on the x-axis.

The control chart is a graph used to study how process changes over time. A control chart always has a central line for average, an upper line for upper control limit, and lower line for the lower control limit. Selection of appropriate control chart is very important in control charts mapping, otherwise ended up with inaccurate control limits for the data. Attribute control charts are used for attribute data. In other words, the data that counts the number of defective items or the number of defects per unit.

For example number of tubes failed on a shop floor. Unlike variable charts, only one chart is plotted for attributes. There are basically four types of control charts that exist for attribute data. The np chart reflects integer numbers rather than proportions.