I performed a qPCR with a dilution series (1:2; 1:4; 1:8; 1:16) to calculate amplification efficiency of the target. My results showed that between two points the difference in Ct values were: 1.6; 0.5; 1.3; 0.2. What could be the reason for this. Shouldn't there be 0.9 - 1.1 the difference between two points? Thank you for your help.