i need create dichtomized variable based on 2 factors (one hopes it's possible).   let's have data:       d <- data.frame (      agegroup = c(2,1,1,2,3,2,1,3,3,3,3,3,1,1,2,3,2,1,1,2,1,2,2,3) ,     gender = c(2,2,2,2,2,2,1,2,1,1,1,2,1,1,2,2,1,1,1,1,2,1,1,1) ,      hourwalking = c(0.3,0.5,1.1,1.1,1.1,1.2,1.2,1.2,1.3,1.5,1.7,1.8,2.1,2.1,2.2,2.2,2.3,2.4,2.4,3,3.1,3.1,4.3,5)             )   i create binary (lowwalkhrs) using gender- , agegroup-specific median (e.g., when agegroup = 1 , gender = 1, median = 2.1 (median found using excel)). lowwalkhrs added variable in dataset, output be:        agegroup gender hourwalk lowwalkhrs         2       2       0.3       1         1       2       0.5       1         1       2       1.1       0         2       2       1.1       1         3       2       1.1       1         2       2       1.2       0         1       1       1.2       1           ....         3       1       5         0   i have rather large dataset (~10k observations),...