Statistics 350 Final Studying Help!

I'm trying to study for my stat 350 final that is tomorrow, i'm getting through the stuff but have ran into a problem that i hardly know where to start. If someone could do what they can to help me it would be awesome!!

The duration of Alzheimer's disease ranges from 3 to 20 years. The average is 8 years and the standard deviation is 4 years. For a clinical study 30 patients are randomly selected that have been determined to be at the very beginning stage of the disease.

a) What is the probability that the average duration of the disease among the sampled patients will be less than 7 years?

b) What is the probability that the average duration of the disease among the sampled patients will be between 7 and 9 years?


Thanks in advance for any help that can be given!

So, I think I may have found the correct answer, so maybe someone could let me know whether this sounds like I did the right thing.

for a) I found that the z value is -1/4, and with a 0.00 confidence interval that puts the probability between .3821 and .4207.

for b) i knew that less than 7 was from a) and found that if supposed to be less than 9 years it would have been between .5793 and .6179, then subtracted 7 from 9 to get between 7 and 9 the probability would be .1972.

Is this the right way to solve this problem?

It's been like 3 years since I did statistics, so I can be of no help.

Perhaps you should post it on a math forum for a more instant result?

http//www.talkstats.com/showthread.php?t=8477

( Hopefully I can find out by 3 or so tomorrow afternoon...

Thanks anyways chex, +K for the thought!

If the sample size is known you should be using the Student's t-score instead of the Z-score. Z-score is used when population standard deviation is known or you are using a proportion and do not know sample size. However, since the sample size is known, you can use the =TDIST function with the df (n-1)=29.

I just took stats this past semester and pretty much mentally flushed everything after the final, sorry I can't be of more help.

The answer is 0.7 for both, but you must [i6b01ff9797]never ask me how I came to that answer[/i6b01ff9797].

uggh, I did so bad in statistics. i think I got like a 50 or something. I thought I could just breeze through, so I didnt put much effort in. oh well