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Confidence Interval Sample Calculation:<br />

Determine 90% confidence interval <strong>for</strong> RT3 Total Nitrogen<br />

X Z<br />

σ<br />

n<br />

± ( α /2)<br />

For 90% α = 1-0.9 = 0.1<br />

Using the t-Table<br />

α /2 = 0.05 (this is called p <strong>for</strong> the table so p = 0.05)<br />

degrees <strong>of</strong> freedom (df) = sample size – 1, or n – 1 df = 7-1 = 6<br />

= 7.8 ± (1.9432)<br />

7.094<br />

7<br />

= 7.8 5.2 ±<br />

There is 90% confidence that the mean is between 2.6 <strong>and</strong> 13.0.<br />

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