Download - 2. Error
9/5/2012
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Why measure errors?
1) To determine the accuracy of numerical results.
2) To develop stopping criteria for iterative algorithms.
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True Error
Defined as the difference between the true value in a calculation and the approximate value found using a numerical method etc.
True Error = True Value – Approximate Value
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Example—True Error
The derivative, )(xf of a function )(xf can be
approximated by the equation,
h
xfhxfxf
)()()('
If xexf 5.07)( and 3.0h
a) Find the approximate value of )2('f
b) True value of )2('f
c) True error for part (a)
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Example (cont.)
Solution:
a) For 2x and 3.0h
3.0
)2()3.02()2('
fff
3.0
)2()3.2( ff
3.0
77 )2(5.0)3.2(5.0 ee
3.0
028.19107.22 263.10
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Example (cont.) Solution:
b) The exact value of )2('f can be found by using
our knowledge of differential calculus. xexf 5.07)(
xexf 5.05.07)(' xe 5.05.3
)2(5.05.3)2(' ef
So the true value of )2('f is
5140.9
True error is calculated as tE True Value – Approximate Value
722.0263.105140.9
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Relative True Error
Defined as the ratio between the true error, and the true value.
Relative True Error ( t ) =
True Error
True Value
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Example—Relative True Error
Following from the previous example for true error,
find the relative true error for xexf 5.07)( at )2('f
with 3.0h
722.0tE
From the previous example,
Value True
ErrorTruet
Relative True Error is defined as
5140.9
722.0 075888.0
as a percentage, %100075888.0 t %5888.7
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Approximate Error
What can be done if true values are not known or are very difficult to obtain?
Approximate error is defined as the difference between the present approximation and the previous approximation.
Approximate Error ( aE ) = Present Approximation – Previous Approximation
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Example—Approximate Error
For xexf 5.07)( at 2x find the following,
a) )2(f using 3.0h
b) )2(f using 15.0h
c) approximate error for the value of )2(f for part b)
Solution:
a) For
h
xfhxfxf
)()()('
2x and 3.0h
3.0
)2()3.02()2('
fff
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Example (cont.)
3.0
)2()3.2( ff
Solution: (cont.)
3.0
77 )2(5.0)3.2(5.0 ee
3.0
028.19107.22 263.10
b) For 2x and 15.0h
15.0
)2()15.02()2('
fff
15.0
)2()15.2( ff
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Example (cont.)
Solution: (cont.)
15.0
77 )2(5.0)15.2(5.0 ee
15.0
028.1950.20 8800.9
c) So the approximate error, aE is
aE Present Approximation – Previous Approximation
263.108800.9
38300.0
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Relative Approximate Error
Defined as the ratio between the approximate error and the present approximation.
Relative Approximate Error ( Approximate Error
Present Approximation a ) =
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Example—Relative Approximate Error
For xexf 5.07)( at 2x , find the relative approximate
error using values from 3.0h and 15.0h
Solution:
From Example 3, the approximate value of 263.10)2( f
using 3.0h and 8800.9)2( f using 15.0h
aE Present Approximation – Previous Approximation
263.108800.9
38300.0
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Example (cont.)
Solution: (cont.)
a
Approximate Error
Present Approximation
8800.9
38300.0 038765.0
as a percentage, %8765.3%100038765.0 a
Absolute relative approximate errors may also need to be calculated,
|038765.0| a %8765.3or038765.0
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How is Absolute Relative Error used as a stopping criterion?
If sa || where s is a pre-specified tolerance, then
no further iterations are necessary and the process is stopped.
If at least m significant digits are required to be correct in the final answer, then
%105.0|| 2 m
a
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Table of Values
For xexf 5.07)( at 2x with varying step size, h
0.3 10.263 N/A 0
0.15 9.8800 3.877% 1
0.10 9.7558 1.273% 1
0.01 9.5378 2.285% 1
0.001 9.5164 0.2249% 2
h )2(f a m
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Propagation of Errors
In numerical methods, the calculations are not made with exact numbers. How do these inaccuracies propagate through the calculations?
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Example 1: Find the bounds for the propagation in adding two numbers. For example if one is calculating X +Y where
X = 1.5 ± 0.05 Y = 3.4 ± 0.04
Solution Maximum possible value of X = 1.55 and Y = 3.44
Maximum possible value of X + Y = 1.55 + 3.44 = 4.99
Minimum possible value of X = 1.45 and Y = 3.36.
Minimum possible value of X + Y = 1.45 + 3.36 = 4.81
Hence
4.81 ≤ X + Y ≤4.99.
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Propagation of Errors In Formulas
f nn XXXXX ,,.......,,, 1321
f
n
n
n
n
XX
fX
X
fX
X
fX
X
ff
1
1
2
2
1
1
.......
If is a function of several variables
then the maximum possible value of the error in is
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Example 2:
The strain in an axial member of a square cross-section is given by Given Find the maximum possible error in the measured strain.
Eh
F2
N9.072Fmm1.04hGPa5.170E
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Example 2:
)1070()104(
72923
610286.64
286.64
EE
hh
FF
Solution
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Example 2:
EhF 2
1
Eh
F
h 3
2
22Eh
F
E
EEh
Fh
Eh
FF
EhE
2232
21
9
2923
933923
105.1)1070()104(
72
0001.0)1070()104(
7229.0
)1070()104(
1
3955.5
Thus
Hence
)3955.5286.64(
Example 3:
Subtraction of numbers that are nearly equal can create unwanted inaccuracies. Using the formula for error propagation, show that this is true.
Solution
Let
Then
So the relative change is
yxz
yy
zx
x
zz
yx )1()1(
yx
yx
yx
z
z
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Example 3:
For example if 001.02x
001.0003.2 y
|003.22|
001.0001.0
z
z
= 0.6667
= 66.67%
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Additional Resources
For all resources on this topic such as digital audiovisual lectures, primers, textbook chapters, multiple-choice tests, worksheets in MATLAB, MATHEMATICA, MathCad and MAPLE, blogs, related physical problems, please visit
http://numericalmethods.eng.usf.edu/topics/measuring_errors.html
THE END
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