formula
an object of class "formula": a symbolic description of the model to be fitted. If the input data is from SEER*Stat, the formula can be ~Year. Otherwise, the full formula should be specified as: ~Alive_at_Start + Died + Lost_to_Followup + Expected_Survival_Interval + Interval + Year
data
an optional data frame, list or environment (or object coercible by as.data.frame to a data frame) containing the variables in the model. If not found in data, the variables are taken from environment(formula).