Why is the data produced with a western blot typically considered to be semi-quantitative?

Western blot data is typically considered semi-quantitative because it offers a relative comparison of protein levels rather than precise measurements of quantity. This is due to two main reasons. Firstly, alterations in loading and transfer rates among samples in different lanes or on separate blots need to be accounted for before accurate comparisons can be made. Secondly, the signal generated by detection does not provide a direct measure of protein quantity across samples. Therefore, attempting to model concentration based on this signal is not recommended.