In the comparative or ΔΔCt method of qPCR data analysis, the Ct values obtained from two different experimental RNA samples are directly normalized to a housekeeping gene and then compared. This method assumes that the amplification efficiencies of the gene of interest and the housekeeping genes are close to 100 percent (meaning a standard or calibration curve slope of -3.32) First, the difference between the Ct values (ΔCt) of the gene of interest and the housekeeping gene is calculated for each experimental sample. Then, the difference in the ΔCt values between the experimental and control samples ΔΔCt is calculated. The fold-change in expression of the gene of interest between the two samples is then equal to 2^(-ΔΔCt).
See Critical Factors for Successful Real-Time PCR for more information.