Hi all,

I have Illumina sequencing results of a bacterial genome and a quality score of 35.89 is associated with these data. I know that a quality score of 30 is 99.99% of base calling accuracy based on [this][1] but what about the meaning of 35.89? Is there a scale used for quality score?

Thank you very much for your much appreciated help on this!


  [1]: https://www.illumina.com/documents/products/technotes/technote_Q-Scores.pdf
The phred score describes the probability of a base call being incorrect. [Wikipedia](https://en.wikipedia.org/wiki/Phred_quality_score) has a great explanation including the formula, which is  `-10xlog10(probability of being wrong)`. 

```
phred = 35.89 # = -10*log10(probability)
35.89/-10 = -3.589
probability = 10^-3.589 = 0.0002576321
```
Therefore, a base call with a Phred score of 35.89 has a 0.02% chance of being wrong. Or, in other words, 0.02% of bases with a Phred score of 35.89 will have been wrongly assigned.