Accuracy And Precision
Accuracy and Precision
Accuracy:
Accuracy of a measuring system is
defined as the closeness of the instrument output to the true value of the
measured quantity.
Precision:
Precision is defined as the ability
of the instrument to reproduce a certain set of readings within a given
accuracy
Comparison:
|
BASIS FOR COMPARISON
|
ACCURACY
|
PRECISION
|
|
Meaning
|
Accuracy
refers to the level of agreement between the actual measurement and the
absolute measurement.
|
Precision
implies the level of variation that lies in the values of several
measurements of the same factor.
|
|
Represents
|
How
closely results agree with the standard value?
|
How
closely the results agree with one another?
|
|
Degree
|
Degree
of conformity
|
Degree
of reproducibility
|
|
Factor
|
Single
factor
|
Multiple
factors
|
|
Measure of
|
Statistical
bias
|
Statistical
variability
|
|
Concerned with
|
Systematic
Error
|
Random
Error
|
Differences
between Accuracy and Precision:
|
Sl.
No.
|
Accuracy
|
Precision
|
|
1
|
It
is the closeness with true value of the quantity being measured
|
It
is measure of reproducibility of the measurements
|
|
2
|
The
accuracy of the measurements means conformity to truth
|
The
term precise means clearly or sharply defined
|
|
3
|
Accuracy
can be improved
|
Precision
cannot be improved
|
|
4
|
Accuracy
depends upon simple techniques of analysis
|
Precision
depends upon many factors and requires many sophisticated techniques of
analysis
|
|
5
|
Accuracy
is necessary but not sufficient condition for precision
|
Precision
is necessary but not sufficient condition for Accuracy
|
Factors
affecting the accuracy of the Measuring System:
The
basic components of an accuracy evaluation are the five elements of a measuring
system such as:
• Factors affecting the
calibration standards
• Factors affecting the
workpiece.
• Factors affecting the
inherent characteristics of the instrument.
• Factors affecting the
person, who carries out the measurements,
• Factors affecting the
environment.
1.
Factors affecting the Standard:
It
may be affected by:
·
Coefficient of thermal expansion
·
Calibration interval
·
Stability with time
·
Elastic properties
·
Geometric compatibility
2.
Factors affecting the Work piece:
These
are:
·
Cleanliness
·
Surface finish, waviness, scratch,
surface defects etc.,
·
Hidden geometry
·
Elastic properties, adequate datum on
the workpiece
·
Arrangement of supporting workpiece
·
Thermal equalization etc.
3.
Factors affecting the inherent characteristics of Instrument:
·
Adequate amplification for accuracy
objective
·
Scale error
·
Effect of friction, backlash,
hysteresis, zero drift error
·
Deformation in handling or use, when
heavy work pieces are measured
·
Calibration errors
·
Mechanical parts (slides, guide ways or
moving elements)
·
Repeatability and readability
·
Contact geometry for both work piece and
standard.
4. Factors affecting person:
·
Training, skill
·
Sense of
precision appreciation
·
Ability to
select measuring instruments and standards
·
Sensible
appreciation of measuring cost
·
Attitude towards
personal accuracy achievements
·
Planning
measurement techniques for minimum cost, consistent with precision requirements
etc.
5. Factors affecting Environment:
·
Temperature,
humidity etc.
·
Clean
surrounding and minimum vibration enhance precision
·
Adequate
illumination
·
Temperature equalization
between standard, work piece and instrument
·
Thermal
expansion effects due to heat radiation from lights
·
Heating
elements, sunlight and people
·
Manual handling
may also introduce thermal expansion
