edgeR normalisation factors different between experimental groups
1
0
Entering edit mode
maltethodberg ▴ 170
@maltethodberg-9690
Last seen 7 days ago
Denmark

I'm analysing an RNA-Seq datasets with edgeR and using calcNormFactors to estimate normalisation factors. In one of my experimental groups all samples have normalisation factors above 1,  while in another group all samples have normalisation factors below 1. Library sizes do not vary systematically between the groups.

What could be an explanation for this pattern?

edger • 797 views
ADD COMMENT
5
Entering edit mode
Aaron Lun ★ 28k
@alun
Last seen 13 hours ago
The city by the bay

This is a standard effect of composition bias - see Robinson and Oshlack (2010) for more details. More genes are upregulated (and/or with larger log-fold changes) in one group compared to the other, resulting in greater composition bias whereby the coverage of all other non-DE genes is suppressed during sequencing. This leads to consistently smaller normalization factors for all samples in the first group, to counter the suppressive effect.

Furthermore, note that the returned normalization factors average to 1 across all samples (for ease of interpretation, and to ensure that the effective library sizes are on the same scale as the originals). This naturally means that if the factors for one group are below 1, the factors for the other group must be above 1.

I wouldn't worry about it.

ADD COMMENT

Login before adding your answer.

Traffic: 620 users visited in the last hour
Help About
FAQ
Access RSS
API
Stats

Use of this site constitutes acceptance of our User Agreement and Privacy Policy.

Powered by the version 2.3.6