document.write( "Question 1141128: You want to estimate the mean time college students spend watching online videos each day. The estimate must be within 1 minute of the population mean, and you want to be 99% confident in your estimate. What sample size would be required if the standard deviation of viewing time is 2.4 minutes? \n" ); document.write( "

Algebra.Com's Answer #761855 by Boreal(15235) $\"\"$ $\"About$

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want a t-test but estimate first with a z, since the z (0.995) is the same regardless of the sample size, whereas the t-value changes.
\n" ); document.write( "error=t*s/sqrt(n) use 2.576 for z in place of t
\n" ); document.write( "1=2.576*2.4/sqrt(n)
\n" ); document.write( "square both sides and multiply through by n
\n" ); document.write( "n=38.22 or 39
\n" ); document.write( "now try tdf=38,0.995 which is 2.72
\n" ); document.write( "2.72*2.4/sqrt(39)=1.04, too big.
\n" ); document.write( "and with a few iterations, a sample size of 42 will be found to give an error of 0.9998, under 1. \n" ); document.write( "

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