diff --git a/R/difft.R b/R/difft.R
index 0ef722d..fbf6740 100644
--- a/R/difft.R
+++ b/R/difft.R
@@ -3,24 +3,33 @@
 #' 
 #' A reference class to represent difference between twin1 and twin2
 #' 
-#' Difft are calculated depending on the favorable outcome chosen. It is the
-#' second level of the outcome. For example, if the outcome is 0 and 1, the
-#' favorable outcome is 1. Then, \deqn{difft_i = twin1_i - twin2_i IF T_i =
-#' 1} \deqn{ difft_i = twin2_i - twin1_i IF T_i = 0}
+#' Difft are calculated depending on the favorable outcome chosen. It is the 
+#' second level of the outcome. For example, if the outcome is 0 and 1, the 
+#' favorable outcome is 1. Then, \deqn{difft_i = twin1_i - twin2_i if T_i = 1}
+#' \deqn{ difft_i = twin2_i - twin1_i if T_i = 0}.
+#' So \emph{absolute} method is :
+#' \deqn{P(Y = 1 | T = 1) - P(Y = 1 | T =0)}
+#' So \emph{relative} method is :
+#' \deqn{P(Y = 1 | T = 1)/P(Y = 1 | T =0)}
+#' So \emph{absolute} method is :
+#' \deqn{logit(P(Y = 1 | T = 1)) - logit(P(Y = 1 | T =0))}
 #' 
 #' @include object.R
 #'   
 #' @field vt.object VT.object (refClass) representing data
 #' @field twin1 vector of \eqn{E(Y|T = real treatment)}
 #' @field twin2 vector of \eqn{E(Y|T = another treatment)}
+#' @field method Method available to compute difft : c("absolute", "relative",
+#'   "logit"). Absolute is default value. See details.
 #' @field difft vector of difference between twin1 and twin2
 #'   
 #' @name VT.difft
-#'  
+#'   
 #' @export VT.difft
-#'  
-#' @seealso \code{\link{VT.forest}}, \code{\link{VT.forest.one}}, \code{\link{VT.forest.double}}
-#' 
+#'   
+#' @seealso \code{\link{VT.forest}}, \code{\link{VT.forest.one}},
+#'   \code{\link{VT.forest.double}}
+#'   
 #' @import methods
 VT.difft <- setRefClass(
   Class = "VT.difft",
@@ -29,16 +38,21 @@ VT.difft <- setRefClass(
     vt.object = "VT.object",
     twin1 = "vector",
     twin2 = "vector",
+    method = "character",
     difft = "vector"
   ),
   
   methods = list(
-    initialize = function(vt.object = VT.object(), ...){
+    initialize = function(vt.object = VT.object(), method = "absolute", ...){
       
       .self$vt.object <- vt.object
       
       .self$twin1 <- .self$twin2  <- rep(NA, nrow(vt.object$data))
       
+      if(!method %in% c("absolute", "relative", "logit")) stop(sprintf("Method %s is not valid", method))
+      
+      .self$method <- method
+      
       .self$initFields(...)
     },
     
@@ -47,7 +61,14 @@ VT.difft <- setRefClass(
       
       if(sum(is.na(.self$twin1)) != 0 | sum(is.na(.self$twin2)) != 0 ) stop("Twins must be valid")
       
-      .self$difft <- ifelse(.self$vt.object$data[, 2] == 1, .self$twin1 - .self$twin2, .self$twin2 - .self$twin1)
+      if(.self$method == "absolute")
+        .self$difft <- ifelse(.self$vt.object$data[, 2] == 1, .self$twin1 - .self$twin2, .self$twin2 - .self$twin1)
+      if(.self$method == "relative")
+        .self$difft <- ifelse(.self$vt.object$data[, 2] == 1, .self$twin1/.self$twin2, .self$twin2/.self$twin1)
+      if(.self$method == "logit"){
+        logit <- function(x) log(x/(1-x))
+        .self$difft <- ifelse(.self$vt.object$data[, 2] == 1, logit(.self$twin1) - logit(.self$twin2), logit(.self$twin2) - logit(.self$twin1))
+      }
       
       return(invisible(.self$difft)) # To see later
     }