Statistics - Change in Volume

I'm not entirely sure what you are asking. Are you asking about standard deviation in estimations?

http://en.wikipedia.org/wiki/Standard_deviation

We need more information on your survey. Was the survey given to the same people in both legs (test and control), or do you have two independent groups - one for each portion of the survey? What do you mean, exactly, by "survey"? Do you mean you surveyed purchasers of your products to determine when they made their purchase, or are you using the term to be synonymous with "study period"? How much data do you have? If you only have two numbers, say 1,000 and 1,200 and want to know if they are significantly different then your denominator becomes critical. 1,000/???? vs 1,200/?????

Let me exaplain it in a different way: on the control leg, the average number of units purchased (claimed) is 1.14. On the test leg, the average is 1.30. How do I find out if the increase on the test leg is statistically significant?
Hope this is clearer. Thanks!

Assuming you know what your sample sizes and Std deviations are you can do a 2-sample t test. Do you have the raw data, or just the summary claims? Did they give you a confidence interval around those means?

Sounds like you are asking for delta (change) percent. If so, the formula is change/base.