@article{charlson1987,
  title={A new method of classifying prognostic comorbidity in longitudinal studies: Development and validation},
  author={Mary E. Charlson and Peter Pompei and Kathy L. Aleš and C. Ronald MacKenzie},
  year={1987},
  journal={Journal of Chronic Diseases},
  doi={10.1016/0021-9681(87)90171-8},
  url={https://doi.org/10.1016/0021-9681(87)90171-8}
}