# Propagating the Error When Taking the Mean of 2 Values - How??

#### johnemel

##### New Member
Hi Everybody

This is my first post. I'm a simple biologist and a few months ago I was completely lame at statistics but came across this very helpful webpage on error.

http://teacher.nsrl.rochester.edu/Phy_labs/AppendixB/AppendixB.html

The part I need for my stats is error propagation, and have been using the formulas at the bottom of the page ok. One thing I'm not clear on though is this. What I am now doing is taking the mean value from 2 experiments. These values are already means and have errors of their own. Calculating the mean is an addition divided by 2, so I have used the formula 4:

(§Z)²= (§A)² + (§B)²

§Z is st error for mean of A and B
§A and §B are errors for values A and B

I calculated §Z using this formula then divided by 2 to get my final standard error. This gives me a nice small(ish) propagated error but I have the feeling this isn't the correct way of doing it. Does one of you stats buffs know the correct calculation?

Here are my actual figures:

Expmnt 1 - 3.74 +/- 1.42
Expmnt 2 - 3.76 +/- 0.09

My calculated mean - 3.75 +/- 0.71