AN INTRODUCTION TO CHART

ByMRINMOY PRATIM BHARADWAZ

SHUBHANKAR SHARMAM.Sc Biostatistics & Demography

2015-17

WHAT IS SQC ?Statistical quality control is simply a statistical method for determining the extent to which quality goals are being met without necessarily checking every item produced and for indicating whether or not the variations which occur are exceeding normal expectation. S.Q.C. Also enables us to decide whether to reject or accept a particular product. (by Grant)

ATTRIBUTES

TECHNIQUES OF S.Q.C.

PROCESS

CONTROL(BY

control chart)

PRODUCT

CONTROL

(BY sampling inspection

plan)VARIABLE

SATTRIBUT

ESVARIABLE

S

-CHART

R-CHART

σ- CHART

C-CHART

NP-CHART

P-CHART

Why chart ?In any measurable quality characteristics can be

applied.To control the measure of location(i.e. mean) this

type of chart is used.

BASIC COMPONENTS OF-chart

A center line(CL), usually the mathematical average of all the samples plotted;

Lower control limits(LCL) and upper control limits(UCL) defining the constraints of MEAN cause variations;

Sample means are plotted in order to control the mean value of a variable.

CONTROL LIMITS • Case 1 : when mean(µ) & s.d.(σ) are known

UCL = µ+A σ

CL =

LCL = µA σWhere μ =mean of the variable,

σ =standard deviation of the variable. A = ; n=2,3…25

Case 2 : when mean(μ) & s.d.(σ) are not knownUCL = +A2

CL =

LCL = - A2 Where =estimated mean of the variable,

=estimated standard deviation A2 = ;n=2,3…25d2=constant depends on “n”

(A2 & values can be get from table on the basic of “n”)

CRITERION FOR DETECTING LACK OF CONTROL IN

• A point outside of the control limits• A run of seven or more points• Points too close to CL• Presence of trends• Presence of cycle

INTERPRETATIONSL NO

SITUATION INTERPRETATION

1 Points beyond limits only on one side

Level of process has shifted

2 Points beyond limits on both side

Level of process is changing in unpredictable manner i.e. variability has increased

3 Out of control on one side

Both level and variability has changed

4 Run of 7 or more points on one side of

CL

Shift in process level

5 Trend of 7 or more points within the

control limits & no points outside control

limits

Process level is gradually changing

6 Points too close to CL Systematic difference in the sub-groups

EXAMPLE

TREND

CYCLIC

MIXTURE

SHIFT

Reference• Fundamentals of applied statistics; S.C.

Gupta; V.K.Kapoor• Introduction to statistical quality control

6th edition; D.C. Montgomery