The length of pregnancies is normally distributed with mean of 268 days and standard deviation of 15 days.
a. One classical use of the normal distribution is inspired by a letter to “Dear Abby” in which a wife claimed to have given birth 308 days after a brief visit from her husband, who was serving in the Navy. Given this information, find the probability of a pregnancy lasting 308 days or longer. What does this suggest?

HELP PLEASE ITS FOR AN ASSIGNMENT.

Just use the formula:





Look this value up in the z table and subtract from 1.

We subtract from 1 since it says, "308 days OR GREATER".

The technique here is to first determine the "z-score" for a 308-day pregnancy, and then determine the probability for an event occurring wit a z-score equal to or greater than this. The Z-score is simply the number of standard deviations away from the mean the number is. Here you are told that the mean is 268, and the standard deviation is 15. Now, 308 days is 40 days greater than the mean, for a Z-score of 40/15 = 2.667. Somewhere in you text book there should be a table of cumulative probabilities for the normal distribution. These are often presented with the cumulative distribution calculated from the mean up to the given Z-score. In other words, the probability that an event is greater than the mean but less than your z-score. So these tables typically start with a probability of 0 for z=0, and work up to a probability of 0.5 for z = 5 or so (post back if your table presents it differently). You want the probablity that the event is greater than the z-score of +2.666. So you subtract the cumulative probability as printed in the text book for z=2.667 from 0.5, and that's your answer.

Post back and tell you what you get for an answer.

Thanks guys I think my problem is the table I'm not 100% on reading them which makes this assignmnet really hard. I've got heaps of questions to go that I need help in. But if you guys know a link that explains the tabels that would be awsome.

Thank you

Since tables can look different, that would be hard to explain without seeing what yours looks like. (I've seen about 3 different ones myself.) If you could find one online that looks exactly like the one in your book, then someone could take a peak and explain how it works.

In fact, here:
z-score table - Google Images
Surely you'll find yours in there somewhere. The first two pictured are the most common ones I see. Quite personally, I like the second one better, but probably only because that's what I got used to. :-)

I've also got another question which has a mean of 38 and SD of 5 then the question asks ' what is the probability that the time it takes to go to into effect is less then 35 min'?
So would you subtract 38 from 35 then divide by 5 giving you 0.6?
After that I don't know what to do? The question asks...


Researches from developing firm have done studies and found that time lapses between administration of the drug and beginning effects of the drug is normally distributed with mean of 38 and SD of 5 minutes.
Q; the drug is administered to one patient selected at random. What is the probability that the time takes to go into effect is less then 35 minutes?
And if the drug administered to a random sample of 10 patients what is the probability that the average time before it is effective for all 10 patients is less then 35 minutes?


Thanks for your help guys my lecturer is useless at teaching me lol