I was interested in differential expression and started to read related materials. They all state that both RNA-seq and microarray data gives relative abundances (I understand that both can be absolute with some reference). As far as I know, this means interval scale instead of ratio scale. Then, they describe that while calculating DE the ratio of two different experiment conditions is used. However, ratio is not valid on interval scale.
What am I missing? Is it not interval scale? Or are there any assumptions that makes these calculations valid?
Thanks for answers!