Question

Samples of ores from the face of a drift on twenty consecutive days and assayed for tungsten (W). The twenty assays average 1.50% W with a variance of 0.1% W^2. Calculate a 90% confidence interval for the tungsten content of the mined mineral. Further mining of the drift would be justified if the grade is above 1.25% W; can we be 95% confident that this is the case ?

Answer 1

The 90% confidence interval for the tungsten content of the mined mineral is:

= 1.50 +- 1.73*(√0.1/20)

= 1.378, 1.622

The 90% confidence interval for the tungsten content of the mined mineral is between 1.378 and 1.622.

The 95% confidence interval for the tungsten content of the mined mineral is:

= 1.50 +- 2.09*(√0.1/20)

= 1.352, 1.648

The 95% confidence interval for the tungsten content of the mined mineral is between 1.352 and 1.648.

We can be 95% confident that mining of the drift would be justified.

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