Hello, I am learning QIIME2 using the official tutorial (“Moving Pictures” tutorial — QIIME 2 2018.8.0 documentation).
There is a step using DADA2 to denoising. The tutorials say " In the
demux.qzv quality plots, we see that the quality of the initial bases seems to be high, so we won’t trim any bases from the beginning of the sequences. The quality seems to drop off around position 120, so we’ll truncate our sequences at 120 bases."
I am confused about how you choose 120. I visualize the demux.qzv using "qiime demux summarize --i-data demux.qza --o-visualization demux.qzv" and I can a pdf plot. However, based on the plot, I can't tell why is 120? There is no 120 on this pdf plot? (see my attached plot)
I can only see the Y-axis has frequency 12? but I don't know what does the frequency 12 mean?