# 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? Commented Aug 23, 2017 at 17:08
• I am testing different treatments to see if they can impact the size of the cell. Commented Aug 24, 2017 at 8:27