Deganveran

02-27-2010, 04:36 PM

I am currently grappling with this question:

In a certain study the average age was 34.79 years. The SD of age was 14.78.

What probability do you have of observing someone 20 years old or younger?

I have determined that 20 years is a z score of 1.06.

However, here is what is throwing me off:

Is that -1.06 or positive?

And when looking at the unit normal table I am wondering if I am looking above Z or below z? I believe it is below z but the unit normal table is saying its 0.8554 or 85.54% chance of finding a 20 year old or younger probability at random. And that, I know is wrong as at least 75% should be within one standard deviation (or z = 1) of the mean.

In a certain study the average age was 34.79 years. The SD of age was 14.78.

What probability do you have of observing someone 20 years old or younger?

I have determined that 20 years is a z score of 1.06.

However, here is what is throwing me off:

Is that -1.06 or positive?

And when looking at the unit normal table I am wondering if I am looking above Z or below z? I believe it is below z but the unit normal table is saying its 0.8554 or 85.54% chance of finding a 20 year old or younger probability at random. And that, I know is wrong as at least 75% should be within one standard deviation (or z = 1) of the mean.