# Factor 2 difference between online sample size calculations - what am I or someone soon to be sorry missing?

#### Forskare

##### New Member
Putting in the same parameters in these two online sample size calculators I get a difference of the sample size by a factor of two. A mean difference of 8, a SD of 17, beta = 0.05 and power = 0.95, the first calculator gives 118 per group, and the second 59.

https://www.sealedenvelope.com/power/continuous-superiority/

https://www.stat.ubc.ca/~rollin/stats/ssize/n1.html

I am guessing the explanation is banale - the second calculation is for one sample compared against a known value, whereas the first is for two groups?

I am assessing an application where the applicant have used the above parameters to dimension a study where they will compare two interventions in two groups (three actually, as two groups will receive TAU, but are assumed to be equally effective) and have concluded that they will need a total of 118 participants, but this is not correct, right? They actually need 118 per group?