C215 - Chapter 6

Question 1

What is Statistical Quality Control (SQC)?

Answer 1

SQC is the term used to describe the set of statistical tools used by quality professionals.

Question 2

What are the three broad categories of Statistical Quality Control?

Answer 2

1. Descriptive statistics
2. Statistical process control (SPC)
3. Acceptance sampling

Question 3

What are Descriptive statistics?

Answer 3

describe quality characteristics and relationships
- simple numbers that summarize data
- mean, standard deviation, range

EX: The average weight of cookies is 50g

Question 4

What is Statistical Process Control (SPC)?

Answer 4

SPC involves inspecting a random sample of the output from a process and deciding whether the process is producing products with characteristics that fall within a predetermined range.

• Using control charts to check if a process is stable or going “out of control.”

EX: Plotting cookie weights on a chart to see if they stay within limits

Question 5

What does SPC determine?

Answer 5

SPC answers the question of whether or not the process is functioning properly :

1. Stable and consistent – The process is working normally, with only natural, small variations.
2. Out of control – The process has unusual variation caused by a problem (like a machine issue, wrong material, or human error).

Question 6

What is Acceptance sampling?

Answer 6

Acceptance sampling is the process of randomly inspecting a sample of goods and deciding whether to accept the entire lot based on the results.

EX: Testing 20 cookies from 1,000 to see if the batch can be shipped.

Question 7

What does Acceptance sampling determine?

Answer 7

Acceptance sampling determines whether a batch of goods should be accepted or rejected.

Question 8

What are the two causes of variation?

Answer 8

1. Common/Natural : Random causes that can’t be identified = normal
2. Special/ Assignable : Causes that can be identified and eliminated = signals something is wrong

Question 9

What is process capability?

Answer 9

The ability of a production process to meet or exceed preset specifications.

•Every process has some natural variation (things aren’t exactly the same every time).
•Process capability tells you whether that variation fits within the customer’s requirements.
• process produces mostly within the limits → capable.
•If it often goes outside the limits → not capable.

“Can the process meet the specs?”

Question 10

What are product specifications/tolerances ?

Answer 10

preset ranges of acceptable quality characteristics, such as product dimensions.

*based on how product is used/expectations

Example: Bottle fill might be 16 oz. ± 0.2 oz. (15.8 oz. – 16.2 oz.)

Question 11

What is the purpose of the process capability index (Cpk)?

Answer 11

assessing capability by evaluating process variability relative to preset product or service specifications.

Measures how well the process fits within spec limits AND whether it is centered

Question 12

What does Process Potential Capability (Cp) assume?

Answer 12

• assumes that the process is centered in the specification range.

-Measures how wide the process spread is compared to the spec limits

Question 13

What does Cpk help to address?

Answer 13

Cpk helps to address a possible lack of centering of the process.

Question 14

What is Six Sigma

Answer 14

Way to measure and improve quality in a process.
The goal is to reduce mistakes , variations and increase satisfaction

Question 15

5 Step Six Sigma Plan
(DMAIC)

Answer 15

D – Define: Identify the problem or process that needs improvement. Set goals.
M – Measure: Collect data to understand how the process currently performs.
A – Analyze: Look at the data to find the root causes of problems.
I – Improve: Implement solutions to fix the problems.
C – Control: Make sure the improvements last over time by monitoring and standardizing the process.

first understand the problem, then measure it, figure out why it happens, fix it, and make sure it stays fixed.

Question 16

What is Acceptance Sampling?

Answer 16

Method to decide whether to accept or reject a batch of products without checking every single item.
Instead of inspecting everything, you inspect a sample and make a decision based on that.

Question 17

What are Control Charts ?

Answer 17

A control chart is a tool used in quality control to monitor a process over time and see if it’s behaving normally

Question 18

Variables Control Charts :
X̄ (X-bar) and R Chart

Answer 18

for measuring averages and range of a process with continuous data (like weight, length, or temperature).

X̄ = average of samples
R = range (difference between highest and lowest in a sample)

• X̄ & R → averages & ranges → numbers you measure

Question 19

Attribute Control Chart :
c-Chart

Answer 19

counts number of defects per item or unit, when defects can occur multiple times in one item.

Example: 3 scratches on one car door

Question 20

Attribute Control Chart:
p-Chart

Answer 20

for proportion of defective items in a batch (pass/fail, yes/no).

Example: 5 defective cookies out of 100