How Do You Calculate Standard Deviation In QPCR?

How Do You Calculate Standard Deviation In QPCR? Also in one of the most used guide to qPCR, “Guide to Performing Relative Quantitation of Gene Expression Using Real-Time Quantitative PCR” (Applied Biosystems), SD is calculated as above mentioned: SD DeltaCt = {(SD of c-myc)^2 + (SD of GAPDH)^2}^1/2 . How do I calculate standard deviation?