Control charts

Question 1

What type of chart would be used to trend data?

Answer 1

Schewhart Chart

Question 2

Draw a Schewhart Chart

Answer 2

Plot in order upper and lower confidence limits should be 3 SD from mean

Question 3

What is the disadvantage/limitation of schewhart charts?

Answer 3

Takes a long time to show a trend.

Question 4

What would indicate a trend or step change on a control chart?

Answer 4

Use nelson rules:
One point above UCL or below LCL
Two points above/below 2 sigma
Nelson Rules state that 2 out of 3 points above/below 2 sigma MUST be above OR below the centerline
Four out of five points above/below 1 sigma
Eight points in a row above/below the center line
Six points in a row ascending or descending (trend)
15 points in a row “hugging” the center line (between -1 and +1 sigma)
14 points in a row alternating up and down
Eight points in a row above 1 sigma or below -1 sigma

Question 5

Explain what a CUSUM is so we are clear that you understand it.

Answer 5

A type of control chart used to detect the deviation of the individual values or subgroup mean from the adjusted target value. In other words, monitor the deviation from the target value.

Question 6

What would you use to analyse skew data?

Answer 6

Interquartile range (IQR)