sampling distributions

djbigsteph said:
i really need help in sampling distributions can you help ?
ok i am having difficulty solving for the sample mean when it is not given. what do i do ?

i also need help with normal cumulative distribution too


TS Contributor
OK. You'll need to post the specific question that you are struggling with, and also include what you've attempted so far...
general electric produces a soft white 100 watt bulb for which it states that the average life is 750 hours. Assume the deviation is 120 hours.

Suppose a consumer agency randomly selects 100 of theses bulbs and finds a sample mean life of 735 hours. should the consumer agency doubt the manufacturer's claim ?

i should first calculate z -score

z=735-750/120 ?

my answer is -.125

please let me know if i am on the right track ?

r uthere ?

sorry i did not know

i checked my z -table and i got .45

.45 is greater than .05 so ho is true ?

i have one more question about range

Suppose scores on a national test follow a normal distribution with mean of 600 and standard deviation of 100.
What sample size would be required such that 95% of the possible sample means would be 20 points from the population mean.
explain how to calculate i am really stuck


TS Contributor
Try not to submit a lot of posts - just put all of your thoughts in one - it makes it a lot easier to follow...this is not the same as IM or chat....

First set up your hypotheses-

Ho: mu = 750
Ha: mu < 750

then set your alpha level (significance level) - usually this is .10, .05, or .01 and is your choice if not specified (usually .05 is chosen)

then compute z:

z = (x - mu) / SE
where SE = s/sqrt(n)

then look up the probability of z in the normal distribution tables - if p(z) is <= alpha, then you have sufficient evidence to reject Ho (reject general electric's claim of mean life of 750 hrs)


TS Contributor
I'm still online, but the site administrator deleted a lot of your posts - don't expect an immediate response just because we're online - often I'm working on something else or away from my computer....

Just post your question and we'll try to get back within a reasonable amount of time - could be immediately, but also could be up to 1-2 days.