# Calculating Sample Size from Power Calculator

I am trying to calculate sample size based on required effect size for my test (site found here: http://www.sample-size.net/sample-size-means/)

My inputs are as follows:

α (two-tailed) = .05

β = 0.100

Split of Test Vs. Control = .5

Desired Effect Size = .03

Standard deviation of the outcome in the population =

My confusion is in regards to this last step - is this the pooled Standard Deviation of my two groups? Not sure what to input here...any help would be appreciated!

As indicated by Stephan, this is the pooled standard deviation. If the effect size that you have specified is a Cohen's d type effect size, then use 1 as your standard deviation. I assume you are aware that this is a very small effect size you are designing your study to detect? You are also designing for a high level of power (1-$\beta$). The two of these combined will result in your needing a very large sample size. Designing for power = .80 is common but this is simply a convention created by Cohen and more power is always better!