The importance of precision
Many scientists consider a two-fold up- or down regulation of a mRNA species as biologically important. Translated into cycles, it means that a one cycle difference between two samples could be interesting. To detect a difference of this magnitude, the precision of your cDNA quantification (the intra-assay variation between identical samples) must therefore be better than one cycle and preferentially as small as possible to mimimise the number of samples needed to obtain a statistically significant result.
Example
We want to demonstrate a one cycle difference between two samples. On one cycler, the precision is +/- 0,5 cycle, which means that triplicates from the two samples would give the results:
Sample 1 Sample 2
19,5 20,5
20 21
20,5 21,5
A t-test would give P =0,07, so we would not be able to demonstrate a difference.
If, however, we used a cycler with a precision of +/- 0.25:
Sample 1 Sample 2
19,75 20,75
20 21
20,25 21,25
we would calculate P=<0,01 and we would be able to demonstrate a difference in target gene expression between the two samples.
