diff --git a/R/chi2Corr.R b/R/chi2Corr.R
index e3bdbbe..b2df6cd 100644
--- a/R/chi2Corr.R
+++ b/R/chi2Corr.R
@@ -3,7 +3,7 @@ chi2Corr <- function(formula, data.obs, namepara1, namepara2, nsimu) {
   ## check validity
   if(nsimu <= 0 | is.na(nsimu) | !is.numeric(nsimu))
     stop("'nsimu' must be a positive number")
-  if(class(formula) != "character")
+  if(!inherits(formula, "character"))
     stop("'formula' must be a string of character")
   if(!is.character(namepara1) | !is.character(namepara2))
     stop("'namepara1' and 'namepara2' must be strings of character")
diff --git a/R/chi2CorrAge.R b/R/chi2CorrAge.R
index 0d0f67a..8ec8038 100644
--- a/R/chi2CorrAge.R
+++ b/R/chi2CorrAge.R
@@ -9,7 +9,7 @@ chi2CorrAge <- function(formula, data.obs, namepara1, namepara2, nameage, w1, w2
     stop("'mort' and 'a' must be numeric")
   if(length(a) != length(mort) + 1)
     stop("'mort' must have the length of 'a' plus 1")
-  if(class(formula) != "character")
+  if(!inherits(formula, "character"))
     stop("'formula' must be a string of character")
   if(!is.character(namepara1) | !is.character(namepara2) | !is.character(nameage))
     stop("'namepara1', 'namepara2' and 'nameage' must be strings of character")
diff --git a/R/chi2CorrGUI.R b/R/chi2CorrGUI.R
index 84ce0bb..b328c8b 100644
--- a/R/chi2CorrGUI.R
+++ b/R/chi2CorrGUI.R
@@ -81,7 +81,7 @@ of serological statuses", font = font2), choose.par)
     
     if(length(ParChoice) != 2)
       tkmessageBox(message = "Please select the two columns of serologic statuses.")
-    else if(class(formul) != "character")
+    else if(!inherits(formul, "character"))
       tkmessageBox(message = "Please enter a model formula.")
     else if(sim <= 0 | is.na(sim) | !is.numeric(sim))
       tkmessageBox(message = "Please enter a positive number of simulations.")