# How do I calculate standard deviation of normalized biological replicates?

I have to start by saying that stats is a foreign language for me. Thus being said, back to my problem.

I am doing experiments with 3 technical replicates and 3 biological replicates. So basically I have 3 wells per experiment and in total 3 experiments from which I want to get a mean and a deviation.

I started by averaging the technical replicates and calculating the SD. Then, I normalized my control to 1 and expressed all other samples (mean and SD) to the initial value of the sample that I set as 1 (so let's say my control is 39=1. Its SD is 3. Then I divided 3/39 and I got the normalized SD of 0.07 and so on for all samples and their SDs that I always divided by 39).

Now, I have 3 independent experiments that I want to average. I have 3 values of 1 for my control and various values for the SD. Same for my samples. I made the average of the individual experiments but I don't know how to calculate the SD. Theoretically, I would calculate the SD of the 3 independent values, but this means that my control will have no SD since all 3 values were set as 1. How should I proceed in this case? Should I average the normalized SDs from each independent experiment?

Thanks! Eveline

• Perhaps it would be better if you told us what the point of your experiment is. What is the scientific question you want your analysis to answer? – mdewey Aug 23 '17 at 17:08
• I am testing different treatments to see if they can impact the size of the cell. – Eveline Aug 24 '17 at 8:27