Hello!

Recently, I'm learning about RNA-seq quatification and I found there are many normalization methods, such as, CMP,RPKM/FPKM,TPM,TMM etc.
After reading some papers about them, I noticed that all of these units mentioned above can't compare directly between samples because they just get relative expression instead of absolute expression.

I have just heard about spike-in method which may not be a proper method to get absolute expression, so, I want to ask you whether there exsits or you have imaged a proper expression unit to get absolute expression or compare between samples.

Thanks for your suggestion!
> compare between samples.

Do you want to do differential expression analysis? If so, the most appropriate tools are DESeq2 and edgeR. These will also deal with normalisation and provide the most sophisticated method so you don't have to worry about absolute vs relative quantification.