I trained a model using a small mri dataset(57 patients). The model's performance was so low(Train set 0.7, Val set 0.7, Test set 0.45).
I found the model segment tumor in upper part of brain well, couldn't segment tumor which is in middle part of brain.
So i stratified sampled the whole dataset with a position stratum(upper, not upper), the model performance was improved.
In this case, is it cheating to do that?