quality control intro + quality chart for variable data

statistical process control (SPC)

verify that business processes are meeting the specifications

• identifies special cause variation

• signals the need to take corrective action when appropriate

control charts

what is the main graphical tool that statistical process control (SPC) relies on?

types of variation in a system

• common causes

• special causes

common causes

what type of variation is described as natural or random and is inherent in any process?

common causes alone

a process is considered in statistical control only if the variation is present due to _____

special causes

non-random (assignable) variation due to identifiable factors

out of control

when special causes of variation are present in a process, the process is deemed to be ______

• it is expensive

• testing can be destructive

• it should be unnecessary for a controlled process

what are three key disadvantages of inspecting every single item (100% inspection?)

statistical sampling

in quality control, what is the more economic alternative to examining an entire lot?

lots

in sampling, the flow of products is broken down into discrete batches called ____

random sample

a sample in which each unit in the lot has an equal chance of being included

• subject it to 100% inspection

• all defective items are repaired or replaced

• return the entire quantity to the supplier

if a random sample from a lot fails a quality test, what are three potential actions for the entire lot?

variable data (quantitative data)

product or service characteristics that can be measured on a continuous scale

EX] length, weight, time

• expressed with statistics such as averages and standard deviations

SPC metrics

• variable data (quantitative data)

• attribute data (qualitative data)

attribute data (qualitative data)

product or service characteristics evaluated with discrete choice

EX] either in or out of tolerance, error/defect present or absent, yes or no, good or bad

• expressed as proportions or rates

Control charts

a specialized run chart that helps an organization track changes in key measures over time

under control

if a sample falls inside the control limits, the process is considered…

• mean

• upper control limit (UCL)

• lower control limit (LCL)

what are the three fundamental lines calculated and plotted on every control chart?

histograms vs control charts

• histograms do not take into account changes over time

• control charts can tell us when a process changes

developing control charts

1. prepare

2. collect data

3. determine control limits

4. analyze and interpret results

5. use a problem-solving tool

6. compute process capability

controlled process

• most points near the center line

• no points are outside control limits

• points fall randomly aboce and below the center line

out-of-control

• instability → points outside control limits

• sudden shift in process average

• cycles

• trends

• nelson rules

nelson rules

the set of guidelines developed by llyod nelson in 1984 for detecting out-of-contol conditions are known as ______

the average value (central tendency) of a variable

what aspect of a process do x-bar charts specifically monitor?

the variation or dispersion among items within each sample

what aspect of a process do R-charts specifically monitor?

X-bar

sum of observations divided by the total number of observations

R-bar

the difference between the largest observation in a sample and the smallest observation

the overall mean which is the average of all the individual sample means

when constructing control charts, what does the x-double bar symbol represent?

the average range which is the average of the ranges from all the individual samples

when constructing control charts, what does the R-bar symbol represent?

the size of the sample (n)

the values for the control chart factors A₂, D₃, and D₄ are determined by what parameter?

R-chart

when constructing and analyzing control charts for variables, which chart must always be analyzed first?

on a daily basis to monitor performance and identify any new special causes that might arise

once a process is confirmed to be in a state of statistical control, how should control charts be used?