I have two datasets on heart rate of subjects that were recorded in two different places (two different continent to be exact). The two research experiments aimed to find the subjects' emotions based on how much their heart rate change over time. I am using machine learning to predict the subjects' emotions and i am getting acceptable result when tested separately on each dataset. However, i get even better result if i merge the two datasets.
I am not however sure if combining the two datasets is acceptable. As I am combining two somehow different datasets, will it create statistical bias? How should i report my finding in a journal paper?