I've seen errors being considered while fitting a model, but not in the source data or when using the fitted data in another calculation. But yeah, I hope it's

I've seen errors being considered while fitting a model, but not in the source data or when using the fitted data in another calculation. But yeah, I hope it's just a simplification for laymen like me. But the fact that apparently there weren't errors bars in a dataset so important such as population estimatives doesn't give me the right impression