# 3. Precision

### 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 mimimize the number of samples needed to obtain a statistically significant result.

### Example

**3.1.** 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: A t-test would give P = 0.07, so we would not be able to demonstrate a difference.

Sample 1 | Sample 2 |

19.75 | 20.75 |

20 | 21 |

20.25 | 21.25 |

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**3.2** If, however, we used a cycler with a precision of +/- 0.25: we would calculate P=0,01 and we would be able to demonstrate a difference in target gene expression between the two samples.