Operations Management: An Integrated Approach, 5th Edition

CONTROL CHARTS FOR ATTRIBUTES

Control charts for attributes are used for quality characteristics that are counted rather than measured. Attributes are discrete in nature and entail simple yes-or-no decisions, for example, the number of nonfunctioning light bulbs, the proportion of broken eggs in a carton, the number of rotten apples, the number of scratches on a tile, or the number of complaints received. Two of the most common types of control charts for attributes are p-charts and c-charts.

P-charts are used to measure the proportion of items in a sample that are defective. Examples are the proportion of broken cookies in a batch and the proportion of cars produced with a misaligned fender. P-charts are appropriate when both the number of defectives measured and the size of the total sample can be counted. A proportion can then be computed and used as the statistic of measurement.

C-charts count the actual number of defects. For example, we can count the number of complaints from customers in a month, the number of bacteria in a petri dish, or the number of barnacles on the bottom of a boat. However, we cannot compute the proportion of complaints from customers, the proportion of bacteria in a petri dish, or the proportion of barnacles on the bottom of a boat. To summarize:

P-charts: Used when observations are placed in either of two groups.

Examples:

• Defective or not defective
• Good or bad
• Broken or not broken

C-charts: Used when defects can be counted per unit of measure. ...