status: Character vector. Name of status variable.
t_risk: Follow-up value to calculate cases, controls. See details.
Returns
KM estimate at specific time point
References
Pencina MJ, D'Agostino RB Sr, Steyerberg EW. Extensions of net reclassification improvement calculations to measure usefulness of new biomarkers. Stat Med. 2011;30(1):11-21