diff --git a/R/calc_npc.R b/R/calc_npc.R
index 5ae4222..dd48e36 100644
--- a/R/calc_npc.R
+++ b/R/calc_npc.R
@@ -21,35 +21,44 @@
 #'
 
 calc_npc <- function(var, N=1, cutoff_divisor=10) {
+    ## only use the first 90% of var - ignore outliers in last 10%
+    l <- length(var)
+    if (l > 0) {
+        l <- as.integer(0.9 * l) 
+        if (l == 0) {
+            l <- len(var)
+        }
+        var <- var[1:l]
+    }
     NPC_DEFAULT <- 4
     d <- var[-length(var)] - var[-1]
     descending_d <- sort(d, decreasing=T)
     max_d <- descending_d[1]
     mean_N_max <- mean(descending_d[1:N])
-    # get measure of spread of data as a proportion of the largest 3 differences
+    ## get measure of spread of data as a proportion of the largest 3 differences
     spread <- mean(d)/mean(descending_d[1:3])*100
-    #print(paste0("spread = ", spread))
     if (spread > 15) {
-        ## data too evenly spread ?
+        ## data too evenly spread for this algorithm to converge?
         return(NPC_DEFAULT)
     } else if (spread > 10) {
         cutoff_divisor <- 5
     }
     cutoff <- mean_N_max/cutoff_divisor
-    #print(paste0("cutoff: ", cutoff))
     groups <- list()
     groups[[1]] <- c(1)
     group_index <- 1
     for (i in 2:length(var)) {
         if (d[i-1] < cutoff) {
-            #print(d[i-1])
-            # include in current group
+            ## include in current group
             groups[[group_index]] <- c(groups[[group_index]], i)
             ## check stopping criteria
             len_curr_group <- length(groups[[group_index]])
             if (len_curr_group > 7 || (i > 10 && len_curr_group > 3)) {
-                #if (len_curr_group > 10) {
                 ## return the last index in the previous group
+                ## catch case where only 1 group
+                if (group_index == 1) {
+                    return(NPC_DEFAULT)
+                }
                 prev_group <- groups[[group_index - 1]]
                 nPC <- prev_group[length(prev_group)]
                 ## not helpful to return nPC=1, so use default in this case
@@ -59,7 +68,7 @@ calc_npc <- function(var, N=1, cutoff_divisor=10) {
                 return(nPC)
             }
         } else {
-            # start new group
+            ## start new group
             group_index <- group_index + 1
             groups[[group_index]] <- i
         }