Measurement Exercise 2
Julie’s Hand Made String Company makes decorative string used for design and craft projects. The products are very successful and highly respected by designers and craftspeople. The string is made by first carding raw cotton to make individual fibers. The quality of these fibers is largely a function of the quality of the raw cotton.

These are then fed by hand into a spinning machine that blends the fibers into one long string. The physical characteristics of the string depending on the feed rate of the fibers and the spinning speed of the machine. These are under the control of the operator. The resulting string is then dyed to the desired color. The mix of the dye and the length of time in the dye determine the final color. Completed and dried string is then wound on bobbins for sale.

The company uses part-time workers, mostly women who need a small job to supplement the family income and enjoy the “craft” aspects of the work. There are two shifts each day, morning and afternoon for four hours each. Workers typically work two or three shifts a week. No worker works on consecutive days and workers are not allowed to work more than one shift a day. To accommodate the workers’ family commitments, workers may work different shifts from day to day or week to week.

Colour and style are the two most important characteristics as the string is used for decoration or design, rarely to actually hold anything together. But the strength of the string is somewhat important as customers don’t want it breaking during use and application. Julie knows her style and color processes are well in control and has almost no complaints or problems reported by customers in these areas. However, she does occasionally have a customer comment about a lack of strength in the string. She has decided to use control charting to help her understand and deal with this issue and has asked for your help.

For her first go at this, Julie takes six samples per hour of string produced at one spinning workstation over a two-day period. She tests the tensile strength of the string and collects the data shown in the table below. How would you approach this problem, what does your analysis of the data suggest to you, and what recommendations would you make to Julie? Remember, Julie is relying on you for advice that will help her improve her business. Your recommendations must be relevant and specific so she can act on them.