<p>A random sample of 119 lightning flashes in a certain region resulted in a sample average radar echo duration of 0.70 sec and a sample standard deviation of 0.49 sec. Calculate a 99% (two-sided) confidence interval for the true average echo duration .</p>
<p>how do i do this? Since we only know the sample standard deviation and not the population SD I cant just do a simple Z score. How do I solve this?</p>
<p>Is the answer (.5843, .8157)? If so, this is just a simple ZInterval, which is easily done by calculator (TI-83 -> Tests > #7). That's how I did it. If you need to do it by hand I'll try to explain it.. in my class we're only required to know how to do it by calculator.</p>
<p>You should use a t - test and not a z test because the population standard deviation is unknown.
t = xbar - u /(standard dev / sqrt n) with degrees of freedom n - 1 so 119-1 = 118
the confidence interval is from xbar - t<em>standdex/sqrt n to xbar + t</em>standdex/sqrt
xbar= .70 s. standard deviation = .49 n =119, degrees of freedom = 118. Plug this onto your calculator under confidence test t-test or somethign like that. I dont have my calculator with you.</p>
<p>hmm we are doing that exact thing right now. But I haven't been doing any of my homework so I don't really know how to do it.</p>