In previous posts (https://statcompute.wordpress.com/2017/01/22/monotonic-binning-with-smbinning-package) and (https://statcompute.wordpress.com/2017/06/15/finer-monotonic-binning-based-on-isotonic-regression), I’ve developed 2 different algorithms for monotonic binning. While the first tends to generate bins with equal densities, the second would define finer bins based on the isotonic regression.

In the code snippet below, a third approach would be illustrated for the purpose to generate bins with roughly equal-sized bads. Once again, for the reporting layer, I leveraged the flexible smbinning::smbinning.custom() function with a small tweak.

df <- sas7bdat::read.sas7bdat("Downloads/accepts.sas7bdat")

monobin <- function(df, x, y) {
  yname <- deparse(substitute(y))
  xname <- deparse(substitute(x))
  d1 <- df[c(yname, xname)]
  d2 <- d1[which(d1[[yname]] == 1), ]
  nbin <- round(1 / max(table(d2[[xname]]) / sum(table(d2[[xname]]))))
  repeat {
    cuts <- Hmisc::cut2(d2[[xname]], g = nbin, onlycuts = T)
    d1$cut <- cut(d1[[xname]], breaks = cuts, include.lowest = T)
    d3 <- Reduce(rbind, Map(function(x) data.frame(xmean = mean(x[[xname]], na.rm = T), ymean = mean(x[[yname]])), split(d1, d1$cut)))
    if(abs(cor(d3$xmean, d3$ymean, method = "spearman")) == 1 | nrow(d3) == 2) {
      break
    }
    nbin <- nbin - 1
  }
  df$good <- 1 -  d1[[yname]]
  return(smbinning::smbinning.custom(df, "good", xname, cuts = cuts[c(-1, -length(cuts))]))
}

As shown in the output, the number of bads in each bin, with the exception for missings, is similar and varying within a small range. However, the number of records tends to increase to ensure the monotonicity of bad rates across all bins.

monobin(df, bureau_score, bad)
#   Cutpoint CntRec CntGood CntBad CntCumRec CntCumGood CntCumBad PctRec GoodRate BadRate    Odds LnOdds     WoE     IV
#1    <= 602    268     136    132       268        136       132 0.0459   0.5075  0.4925  1.0303 0.0299 -1.3261 0.1075
#2    <= 621    311     185    126       579        321       258 0.0533   0.5949  0.4051  1.4683 0.3841 -0.9719 0.0636
#3    <= 636    302     186    116       881        507       374 0.0517   0.6159  0.3841  1.6034 0.4722 -0.8838 0.0503
#4    <= 649    392     259    133      1273        766       507 0.0672   0.6607  0.3393  1.9474 0.6665 -0.6895 0.0382
#5    <= 661    387     268    119      1660       1034       626 0.0663   0.6925  0.3075  2.2521 0.8119 -0.5441 0.0227
#6    <= 676    529     415    114      2189       1449       740 0.0906   0.7845  0.2155  3.6404 1.2921 -0.0639 0.0004
#7    <= 693    606     491    115      2795       1940       855 0.1038   0.8102  0.1898  4.2696 1.4515  0.0956 0.0009
#8     717   1883    1775    108      5522       4431      1091 0.3226   0.9426  0.0574 16.4352 2.7994  1.4435 0.4217
#10  Missing    315     210    105      5837       4641      1196 0.0540   0.6667  0.3333  2.0000 0.6931 -0.6628 0.0282
#11    Total   5837    4641   1196        NA         NA        NA 1.0000   0.7951  0.2049  3.8804 1.3559  0.0000 0.7508