Using Minitab to Curb Supplier Defects

Minitab Blog Editor 13 September, 2012

Workers used statistics and Minitab to identify the source of defective partsSharing how people and companies use Minitab software to improve quality is without a doubt the highlight of my job!

It never ceases to amaze me that a common thread among many organizations is the desire to keep continuous improvement at the forefront of their business. It’s also pretty neat that many of these same organizations trust Minitab to analyze their data and organize their quality improvement projects.

One of my favorite stories involves Skills of Central Pennsylvania. The organization's corporate office is actually located down the road from Minitab’s World Headquarters in State College, Pa.

Skills Workers Find a Defect

Skills is a nonprofit group that provides vocational, rehabilitative, and social opportunities to individuals with disabilities in central Pennsylvania. The organization’s Employment Services Division specializes in matching the needs of employers in the area to the abilities of the workers Skills serves.

The Skills workforce primarily staffs for manufacturing companies, and has become known for their high-quality work and willingness to go above and beyond employer expectations with their service. One example of this came when the Centre Region branch of the Skills workforce noticed that one of their employers' manufacturing processes could be improved.

This group of Skills workers was hired by a local manufacturer to assemble 4 ml. and 2 ml. vials for the production of liquid chromatography columns that are used in chemical laboratories. The vials act as sample vessels for testing different substance molecules to ensure they meet strict industry specifications.

Skills workers assemble the vials by attaching blister caps to vial bottle tops, and  the caps must be properly aligned with threading placed on the sides of the bottles to ensure no liquid can escape.

Shortly after starting the project, the Skills team discovered a defect in the threading of certain vial bottles that caused several blister caps not to fit and tighten properly. In these cases, the bottles with defective threading were discarded and had to be replaced with new bottles. The team wanted to track and report the number of defective units to the vial bottle supplier in order to prevent the manufacturer from continuing to pay for the rejects.

The Assistant Helps Non-Statisticians Analyze Their Data

The Skills team collected data on the number of defective bottles found in each box delivered by the supplier, and then they recorded the number of rejects in each box and entered that information into a Minitab worksheet:

Recording the number of defects in a worksheet for data analysis

Fultz knew she wanted the analysis to help her examine the proportion of defective vial bottles and assess the stability of the process, but she wasn’t quite sure where to begin.

By using Minitab's Assistant menu, she was able to follow an interactive decision tree to determine which analysis to use. Fultz selected the P chart option because she and her team were investigating rejected items, and then she quickly completed a simple dialog box before being presented with not only the P chart itself, but also detailed reports (like the one below) that included an interpretation of the results.

explanation of p chart, proportion of defective items, subgroup size

Project Results

The results of the analysis revealed the proportion of defective vial bottles was not stable, and the control chart easily showed the team the out-of-control subgroups in relation to the defined upper and lower control limits:

P chart reveals proportion of defective items and stability

The results also showed the team that more than 20% of the vials in just one box were defective!

Once they completed their analysis, the Skills team was able to confidently and clearly communicate that there was an issue with the vials to both the manufacturer and the supplier.

Fultz says the manufacturer is grateful that Skills started tracking and analyzing the defective vials. And now that the supplier is aware of the defects, they are working on improving the quality of their processes and providing the manufacturer with boxes of defect-free vial bottles.

To read the full case study on, as well as other case studies, check out our Case Studies page.

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