# sample size for medical statistics?

#### Hanna

##### New Member
Hello! I hope I'm posting in the right place..

I'm doing a research in biomedicine for my bachelor's degree (testing how many embolus a machine can find compared to how many I can find = testing how good the machine is) and i'm trying to calculate which sample size I need for statistical power. I got a note from my supervisor saying "The power calculation is based on Binominal testing. We want to be sure that the machine finds >95% of all the emboli, then we need 59 emboli to be found for p<0.05."

He's telling me, in other words, that I need to have a sample size of 59 emboli for statistical power.. i've tried to trace it down a bit, and found this formula:

n0 = (z^2 *p*q) /(e^2)

from what I understand, z= the z value (1.96 for 95%), p= the percentage I picked (0.5), q is (1-0.5) and e... might be the confidence interval?

My problem is.. I don't know what "e" is, for starters, or how to get a hold of it.. and i can't find any solid proof that "59" really is the number of emboli I need.. and "because he said so" looks very bad in a report =/

Sorry about the textwall hope you can understand what i mean, i'm not a native speaker of English..
Sincerely
Hanna

##### Ninja say what!?!
1) I'm not familiar with that formula. Where did you find that?
2) e is not a variable. It's a number, approximately equal to 2.718 (sort of like pi)
3) You'll have to do a lot of reading to understand what statistical power is and how to calculate sample sizes. Here are some links. The third one will calculate the sample size required for you. I have not verified that it is correct at calculating. Maybe you can verify it while you're learning.