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Rock Mechanics.pdf - Mining and Blasting

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Figure 3.16 Frequency distribution<br />

of the sample mean (after Priest <strong>and</strong><br />

Hudson, 1981).<br />

COLLECTING STRUCTURAL DATA<br />

Table 3.4 Values of ( z) for the normal distribution.<br />

z (z)<br />

0.675 0.50<br />

0.842 0.60<br />

1.036 0.70<br />

1.282 0.80<br />

1.645 0.90<br />

1.960 0.95<br />

2.576 0.99<br />

z <strong>and</strong> (z) can be found in most statistics textbooks. Selected values are given in<br />

Table 3.4.<br />

It will be recalled that discontinuity spacings, x, often follow the negative exponential<br />

probability density function<br />

f (x) = e −x<br />

(3.1)<br />

It so happens that, for this distribution, the mean <strong>and</strong> st<strong>and</strong>ard deviation of the population<br />

are equal. For a sample of size n, the b<strong>and</strong>width of (z) confidence is then<br />

¯x ± (z ¯x)/ √ n. Alternatively, this b<strong>and</strong>width can be written as ¯x ± ε ¯x where ε is the<br />

63

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