# Thread: Normal Probability Distribution Help

1. ## Normal Probability Distribution Help

Hello,

Firstly, I would just like to add the question under discussion as well as some data before I begin:

A machine is set to produce components having a length of $21.15 \pm 0.05cm.$ In a trial, two hundred successive components were measured with the following results:

$Length (cm)\ 21.12\ 21.13\ 21.14\ 21.15\ 21.16\ 21.17\ 21.18\ 21.19$

$Number\ 7\ 11\ 24\ 43\ 47\ 29\ 23\ 16$

1) Plot the data on Normal Probability Distribution paper.

2) State with reason if they may be considered to be Normally distributed.

3) Obtain values for the mean and standard deviation from your graph.

Now, unfortunately, I've been away off-sick for the remaining remanance of my final week of term and I've been going through my book to help me with this problem in order to make up for lost time, and there are some questions I would like to ask before I begin the task in hand.

Q1: How do I determine what my Upper Class Boundaries are? In my book they appear to be just adding 5. So, with respect to the data above, would this yield (In a table against cumulative frequency and Percentage cumulative frequency): less than 21.62, less than 21.63, less than 21.64 and so on in ascending order?

Q2: This is my biggest problem. How does the scale work on Normal Probability distribution paper? I don't understand how the ordinate axis scale works in order to start plotting co-ordinates?

If someone could help me with the above, everything should hopefully be down hill from here. Thank you.

2. ## Re: Normal Probability Distribution Help

It's ok now, no worries guys, I've done it - problem solved.