Sunday, October 1, 2023

SPC interview Questions

SPC (Statistical process control)

Here i am going to discuss about- what is SPC? What is its use? Why SPC is used?
Content added in this post are from training, which i took from an institute.

What is SPC?

SPC stand for Statistical process Control. It is Quality tool which is used to measure and control process variation by using statistics.  Statistics here is set of information derived from sample data.

Well SPC is Core tool of QMS, comprehensive tool published by AIAG. From interview point of view, SPC was introduced by William A shewart in 1924.

What are Uses of SPC?

With SPC we understand process behavior. SPC is used as a tool to develop process, identifying source of variation in process & removing them to sustain better quality. SPC can be use as problem solving tool. By SPC we try to set those manufacturing parameters which give optimum value, by which cost of manufacturing will reduce.

SPC formulas

So SPC is use to study process behavior.  With SPC we develop our process of manufacturing, or not only develop but we strengthen our process by doing SPC. This core tool not only use in automotive sectors, but also use widely in other no
automotive fields, like medical, defense.

Process behavior is understand by shape, spread &Location.  For Location we calculate mean. For Spread we calculate Standard deviation (sigma). For Shape we use help of Histogram.

SPC summary in short is explained in below picture:-


SPC Definition & Variation

SPC is comprises of three words Statistical Process Control. Each word has its meaning.

Statistical: – Interpretation of Data [which we have collected] to estimate process.

Process: – Convert input into output by controlling man, machine, method & material.  

Control: – Keeping process variation within spec.

Variation: – Gap/Dissimilarity between the two objects called variation. Variation can cause unwanted rejection, undesired rework, Customer dissatisfaction. 

Cause of Variation:

1. Common cause- Which will come & cannot be control/stop.

2. Special Cause- Which come sudden [Black Noise, Assignable cause]

Total Variability:-
Process Behavior estimation on the basis of Variation:-
Location: – MEAN [Denoted by X-bar & µ] arithmetical sum of all number divide by total number.
Spread: – Can be check by Range or by Standard deviation Standard deviation (sigma) is calculated by formula. 
Note:- Process variation is 6 time of standard deviation.
Shape:- Can be analysis with the help of Histogram (which will reveal amount of variation that process has in it)
Histogram is basically graphical representation of variation data. By histogram we can also see how process is spreading (we will also use standard deviation for this) 

Types of Histogram

  1. Normal –Bell shape & Normal
  2. Comb like
  3. Positive or negative skewed
  4. Precipice type
  5. Plateau type
  6. Bimodal
  7. Isolated Peak Type
Histogram shape Types
Histogram Shapes Types
Normal distribution shape is considered OK while doing process study on the basis of process distribution,  in normal distribution process variation is within Control limit.
Normal Distribution in SPC study
Normal Distribution curve in SPC
Control Limit is used in Control Chart & Control Chart is drawn after collection of variable observation, under that chart we see how data is being distributed.  
On the basis of variable data we make study to check process capability. 

Types of SPC charts

  1. X-bar & R chart
  2. X-bar & S chart
  3. I & MR chart
  4. P-chart
  5. np &p chart
  6. u-chart
  7. c-chart & u-chart
You can understand by below picture:-
Types of SPC Charts
Types of SPC Chart

A. Variable Control Chart (X (bar) & R chart)

In variable control chart-Most commonly used chart that come in picture is X(bar) & R chart.

For X(bar) & R chart , we have to collect variable data & have to decide followings:-

  1. Decide Subgroup Size
  2. Decide subgroup Frequency  [Don’t confuse-It is just time interval]
  3. Decide number of Sub-group
Sub-group in variable control table
To study X(bar) & R chart, we have to some calculations:-
  1. Calculate average of each subgroup. (X-bar)
  2. Calculate range of each subgroup. (R=Xmax -Xmin)
  3. Calculate average of average of each subgroup (X-Double bar)
  4. Calculate average of Range (R-bar).
  5. Calculate trial control limit for Range chart. (UCLR, LCLR)
  6. Calculate control limit for Average chart. (UCLx, LCLx)
Calculation for Variable Control Chart

Now we have done all calculation for UCL & LCL for X̅ & R.

We decide subgroup size n=5, subgroup number 20.

Calculation for control chart in SPC

To create X̅ Chart, we will hide all Range row, and will select X̅ data and will insert line chart.

Same procedure will be followed to create Range chart.

Range chart in SPC

By control Chart we identify any special cause in process, see unnatural pattern in data.

Any special cause could be identify when observation cross limit lines.

Control chart Interpretation

  1. One point beyond zone A- Caused by large change in process
  2. Seven point in a row on one side of center line-Caused by process mean shift
  3. Seven point in row steadily increasing or decreasing-Caused by Mechanical wear, contamination, Chemical depletion, etc
  4. Fourteen point in a row alternating up & down-Caused by over adjustment, shift to shift variation, machine to machine variation.
  5. Two out of three point in row in same zone or beyond- Caused by Major special Cause variation
  6. Four Out of five point in a row in same zone B and beyond.
  7. Fifteen point in a row in zone C (Above or below centre line)
  8. Eight point in a row on both sides of center line with none in Zone-C

Difference between Specification Limit and Control Limit

Specification limit are product tolerance limit, usually taken from product drawing. Example if diameter of hole in drawing is 8+1/-2 (Min- 6 mm and Max-9mm) this is product specification limit. While control limit is related to process control limit.

There is no relation between specification limit and control limit. 

Control limit is based upon mean and standard deviation. Control limit will be within process width. Process width is mean+3 x standard deviation. Control limit is limit of process, beyond this the variation will come that will affect product quality somewhere.

Cp and Cpk is calculate to study the process capability and process performance.

Cp & Pp – Process Capability & Process Performance

Process Capability (Cp & Cpk) indicates the ability of process to meet specification when process operates under common cause.

In practical situation, it is obvious to show variation due to both common and assignable cause. So we have
to analysis process behavior due to combined effect of common and assignable cause.  We indicate by Pp, Ppk.

Process Capability= Cp

Cp is for process capability. Which is study by process graph. Cp always show spread of curve. Process will be capable if curve is smaller to specification limit and equally distributed to both side of mean.

  1. Calculate process standard deviation (σ=R̅/d2) 
  2. Calculate process Capability (Cp)= (USL-LSL)/6σ

σ= 0.21/2.326 ; (0.090)



USL & LSL are maximum limit & minimum limit for spec:- 0.7±0.2

Maximum:- 0.9 & Minimum-0.5

Process Capability= Cpk

Cpk is for Process performance. It show spread as well as location. 

Minimum value between Cpu & CpL is consider as Cpk 


CpL= (X̿-LSL)/3σ

Cpu= (0.9-0.718)/3*0.09 ; Cpu=0.689

CpL= (0.718-0.5)/3*0.09; Cpu=0.825

Here Cpk value will be consider 0.689 (Since is minimum)

Cp is consider only spread, not the location. While Cpk is consider both spread and location.


Hi.. welcome to blog. Professional Engineer, blogger and tech educator. This blog is being managed since 2020. Purpose of this blog website is to get connected with those people who are interested in learning technical skills and are doing work in industries. Also we create content to help people in need. I am reachable, click on contact page for any support. Thank You

One thought on “SPC interview Questions

Comments are closed.