Question

Assume that the helium porosity (in percentage) of coal samples taken from any particular seam is...

Assume that the helium porosity (in percentage) of coal samples taken from any particular seam is normally distributed.

How large a sample size is necessary if the width of the 90% interval is to be .20? Sample standard deviation of .65

Homework Answers

Answer #1

Solution :

Given that,

standard deviation = =0.65

Margin of error = E = width/2=0.20/2=0.1

At 90% confidence level

= 1 - 90%  

= 1 - 0.90 =0.10

/2 = 0.05

Z/2 = Z0.05 = 1.645 ( Using z table ( see the 0.05 value in standard normal (z) table corresponding z value is 1.645 )  

sample size = n = [Z/2* / E] 2

n = ( 1.645 *0.65 /0.1 )2

n =640.47

Sample size = n =641

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