Week 7 - Statistical Process Charts (SPC)

Question 1

What are the two tools to gather & measure data?

Answer 1

• Check sheets
• Pareto charts

Question 2

What are the different tools to analyse and improve? (4)

Answer 2

• Histograms
• Scatter diagrams
• Cause-and-effect diagrams
• Flowcharts

Question 3

What is the tool for control?

Answer 3

Statistical process control charts

Question 4

When would we describe a process as capable?

Answer 4

If the process naturally spreads plus and minus 3 sigma

Question 5

4 Characteristics of a control chart

Answer 5

• It is ‘normal’ to have some variation in performance
• When the process is normal we can draw control limits based on the distribution
• Points outside the control limits are likely to be associated with the feeling that something should be corrected in the process
• Only 0.27% of points normally fall outside

Question 6

Probability for different improbability points:
- 1 point outside
- 2 successive points
- 7 successive points

Answer 6

• 1 point outside: probability is 0.003 due to an assignable cause
• Two successive points above the standard deviation on the same side of the centre line has a probability of 0.004
• Seven successive points that are all above or below the mean has a probability of 0.007

Question 7

4 different special cause detection rules (causes for investigation)

Answer 7

• One (1) point more than 3 sigmas from center line (outside the control limits)
• A run - seven (7) consecutive points on the same side of center line
• A run - seven (7) consecutive points increasing or decreasing
• Is there a pattern? 14 consecutive points alternating up and down

Question 8

What is different between accuracy and precision?

Answer 8

Precision is how close together the points are on a target and accuracy is how many pints hit the target but tend to be more spread out

Question 9

What is meant by an x-bar bar?

Answer 9

Is the dotted line taken at the end and is the average of the average

Question 10

Formulas for sample means chart (2)

Answer 10

Question 11

Formulas for a range chart (2)

Answer 11

Question 12

What does a six sigma process require?

Answer 12

The process standard deviation be no more than one-twelfth of the total allowable spread

Question 13

What is process sigma/ customer voice?

Answer 13

It measures the performance of a process by the number of standard deviations the customer requirements are from the process mean - theories originated from Dr. Genichi Taguchi

Question 14

What is Cp?

Answer 14

Is the upper specification limit minus the lower specification limit divided by six sigma

Question 15

What is Cpk

Answer 15

Is the lower value of the (USL minus the mean/3σ) or the mean minus LSL/3σ which gives us an idea of how stable the process is in relation to the service limits

Question 16

Formula for Cpk

Answer 16