The charge-to-tap time (min) for carbon steel in one type of open hearth furnace is to be determined for each heat in a sample of size $n$. If the investigator believes that almost all times in the distribution are between 320 and $440,$ what sample size would be appropriate for estimating the true average time to within 5 min. with a confidence level of $95 \%$ ? A reasonable value for $s$ is $(440-320) / 4=30 .$ Thus$$n=\left[\frac{(1.96)(30)}{5}\right]^{2}=138.3$$since the sample size must be an integer, $n=139$ should be used. Note that estimating to within 5 min. with the specified confidence level is equivalent to a CI width of $10 \mathrm{min}$.

Glencoe Algebra 1, Student Edition, 9780079039897, 0079039898, 2018
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ISBN:9780079039897
Author:Carter
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Chapter4: Equations Of Linear Functions
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The charge-to-tap time (min) for carbon steel in one type of open hearth furnace is to be determined for each heat in a sample of size $n$. If the investigator believes that almost all times in the distribution are between 320 and $440,$ what sample size would be appropriate for estimating the true average time to within 5 min. with a confidence level of $95 \%$ ? A reasonable value for $s$ is $(440-320) / 4=30 .$ Thus
$$
n=\left[\frac{(1.96)(30)}{5}\right]^{2}=138.3
$$
since the sample size must be an integer, $n=139$ should be used. Note that estimating to within 5 min. with the specified confidence level is equivalent to a CI width of $10 \mathrm{min}$.

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