Precision of the real-time PCR values (Ct) are assessed by the standard deviation of the Ct values obtained from the replicates. If SD is >0.25, the power to discriminate between a two-fold dilution is less than 95%. If in a ROX requiring instrument, a master mix with lower than required ROX concentration is used, the SD will be large and may be reduced by using an appropriate master mix. See ABI Publication: Understanding Ct Value for more on SD.