Hi jenny1988!

What you are describing is a t-test.

More specifically, an independent two-sample t-test with presumably equal variances.

As an example, after 100 measurements of red balls, and 100 measurements of green balls, your t-score is:

This assumes you have properly determined Y(R) and Y(G).

Then you would have to look it up in a t-test table, specifying df=2x100-2=198.

This will give you the confidence level you ask for.

The proper procedure is:

- Calculate the t-score with the given formula (see here).
- Look up p-value that corresponds with this t-score.

You will need to specify thedegrees of freedom(df) as given in the article.

You can do this with a graphical calculator, SPSS, Excel, an online calculator, or whatnot.- If this p-value is smaller than 0.05, you can say with a confidence of 95% that the diameters are different.

Or with an even smaller p-value, you can say with a confidence of (1-2p)x100% that the diameters are different.