Statistical Quality control - SQC

Question 1

Why we do SQC? Why focus is on variation and how variation manifests itself in
processes?

Answer 1

We do SQC since visual inspection incurs high error rates, we need to have a control system, an implementation of a standard or goal for measuring accomplishment

We focus on variation since it defines if your process is within the limits the supplier or customer defines.

Variation are randoms and occur

Question 2

What is Taguchi principle? How did it change quality thinking?

Answer 2

The taguchi principle focuses on improving the quality of manufactured goods by minimizing variation and aiming for robust design

it changed quality thinking by:
1) Emphasis on variation reduction: Quality management began to prioritize reducing variability within processes rather than just inspecting final products for defects.

2) Cost of quality: highlights the hidden costs of variation and the importance of aiming for the target value rather than simply staying within specification limits.

3)Design of experiments: his methods became widely known, enabling more effective and efficient identification of factors affecting quality

Question 3

How to read control charts and what the different charts mean and why we do
them?

Answer 3

A control chart is a chart with statistically-determined upper and lower control limits called UCL and LCL

On a graph there is the center line where data points are placed a bit everywhere and it is inbetween the UCL and LCL

1) X bar chart => control the evolution of a process

2) Rbar chart => control the variation of a process

3) P bar chart => track the evolution of a proportion

Question 4

6-Sigma principle and what is the main message of it?

Answer 4

Six sigma seeks to improve the quality of process outputs by identifying and removing the causes of defects (errors) and minimizing variability in manufacturing and business processes

it measures defects in million. the higher the number of sigmas, the more we accepts defects and the less defects we have. the defects depends on the clients or supplier

Question 5

What is the main logic of sampling and random sample selection, why it is important and why actual sample size of the sample is surprisingly small despite
the size of the lot-size that is to be inspected?

Answer 5

Small sample sizes still provide accurate results. It’s due to statistical principles (random sampling) and it still gives accurate representation.

Question 6

What is a process in control and capability

Answer 6

A process in control defines if your process is within the limits you (supplier) or customer defines.

Process capability defines if you are able to do what is required. For example: 500 products in 4 days.

It is important to understand that a process in control might still fail to deliver the quality demanded

Question 7

How to know if a process is not under control

Answer 7

1. At least one data point is outside the control limit
2. A sequence of eight subsequent points above or below the process mean
3. A sequence of seven subsequent points going to the same direction
4. A pattern that you can identify