I hope I didn't misunderstand the situation. The *population* has a standard deviation of 8 hours.
(I guess the SD refers to the population, since the sentence came before the sentence "A random sample of 130 ...", and the word "mean" was preceded by "sample" while the word "standard deviation" was not.) The *random sample* has a mean of 11.4 hours. It seems that nothing was said about the standard deviation of the random sample. The solution posted by Dirk Vdm should work. Please correct me if I misinterpret the situation. [EMAIL PROTECTED] (Radford Neal) wrote in news:[EMAIL PROTECTED]: > In article <[EMAIL PROTECTED]>, > Jo <[EMAIL PROTECTED]> wrote: > >>The time that students spend at part time jobs per week is >>approximately Normally distributed with standard deviation 8 hours. A >>random sample of 130 students is taken and the sample mean is 11.4 >>hours. >> >>How do I find the 90% lower confidence limit for the true mean (mu)? > > You don't, since the assumptions stated are false. It's not possible > for the time to have a mean of around 11 and standard deviation of 8, > since that would produce substantial numbers of students who work for > a negative number of hours, which is clearly impossible. > -- Shu Fai CHEUNG (Please replace the string after @ by "psy.cuhk.edu.hk". Sorry for the inconvenience caused.) . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
