diff --git a/ccn2019.Rmd b/ccn2019.Rmd
index 8cb92cc..efba46c 100644
--- a/ccn2019.Rmd
+++ b/ccn2019.Rmd
@@ -105,8 +105,17 @@
   ifelse(stim==lag1 ||  stim==lag3, T, F)
 }
 
+create_history <- function(stims, max=4) {
+  res <- c('')
+  for (i in 2:length(stims)) {
+    res[i] <- str_sub(paste(res[i-1], stims[i], sep=''),-max,-1)
+  }
+  res
+}
 
 NB %>%
+  group_by(participant, condition, block) %>%
+  mutate(history = create_history(stimulus)) %>%
   mutate(
     lure = map_chr(
       stimulus,