Civil Engineering Reference
In-Depth Information
Mean Strength 35, 40, 45; Standard Deviation 4
0.12
0.1
0.08
0.06
0.04
0.02
0
0
10
20
30
40
50
60
70
80
Strength (MPa)
Figure 9.2
Three distributions with the same
σ
and different values of
X
.
Mean Strength 40; Standard Deviation 2, 4, 6
0.25
0.2
0.15
0.1
0.05
0
0
10
20
30 40
Strength (MPa)
50
60
70
80
Figure 9.3
Three distributions with the same
X
and different values of
σ
.
of all results. The published tables relate the area to the value of
k
. Table 9.1
is an extract from such a table.
9.1.1 Permissible percentage defective
There is logic in using a 5% defective level (or even a 10% defective level)
in that adherence to the assumed statistical distribution is not exact. The
assumption predicts reasonably well the level below which 5% of results
fall (in the authors' experience there are likely to be actually 2% to 3%
below the level below which 5% are predicted to fall, but more about this
later) but at the 0.1% level, the assumption has become highly theoretical
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