How does one go about comparing expression levels of one gene across multiple microarray datasets? In my example, I've downloaded the expression levels for my gene of interest from different microarray datasets for different cell populations. How would I pool the datasets together?

Say I have common cells from multiple datasets, would I be able to just combine them into one data frame and visualize them as one or would I need to normalize some aspect of that?

I'm really confused because I've had different people tell me different things.