Take individual characteristics and map it to individual-day observations. For example, individual A/B is 40/45 years old and has 4/2 day observations in dataset D. Therefore, mapping age to dataset D generates 40, 40, 40, 40, 45, 45 for day 1,2,3,4/1,2 observation of A/B respectively.
ind_to_day(x, df)
covariates data for individuals to be merged.
day observation dataset.
individual to day observations.