Methods for obtaining Flory parameter (χ) values for polymer blends or blocks are critically examined, for three simple systems (polystyrene/poly(methyl methacrylate), polystyrene/polycarbonate, poly(methyl methacrylate)/polycarbonate). In particular, uncertainties for calculated and experimental values are discussed and compared. Data obtained using scattering measurements combined with a mean-field analysis are reexamined. We show that different experimental techniques will lead to significantly different χ values for a given system, and that even for one experimental technique, the uncertainty on molecular parameters used to fit the data may impact the extracted value for χ. We also show that, though frequently used in the literature, χ values obtained via solubility parameters should not be used for any quantitative analysis of polymer-polymer interactions. This shows that the Flory parameter may not be relevant when discussing small effects on vague notions such as compatibility between polymers, or should at least be used with caution.