diff --git a/sho/bit.py b/sho/bit.py
index ca84849..02cd5a4 100644
--- a/sho/bit.py
+++ b/sho/bit.py
@@ -55,6 +55,7 @@ def rand(domain_width, nb_sensors):
 
 def neighb_square(sol, scale, domain_width):
     """Draw a random array by moving every ones to adjacent cells."""
+    assert(0 < scale <= 1)
     # Copy, because Python pass by reference
     # and we may not want to alter the original solution.
     new = copy.copy(sol)
@@ -64,7 +65,7 @@ def neighb_square(sol, scale, domain_width):
             # coordinates of images (row,col).
             if sol[py][px] == 1:
                 # Add a one somewhere around.
-                w = scale//2 * domain_width
+                w = scale/2 * domain_width
                 ny = np.random.randint(py-w,py+w)
                 nx = np.random.randint(px-w,px+w)
                 ny = min(max(0,ny),domain_width-1)
diff --git a/sho/num.py b/sho/num.py
index bb5e86c..dcdbffd 100644
--- a/sho/num.py
+++ b/sho/num.py
@@ -50,6 +50,7 @@ def rand(dim, scale):
 def neighb_square(sol, scale, domain_width):
     """Draw a random vector in a square of witdh `scale`
     around the given one."""
+    assert(0 < scale <= 1)
     new = sol + (np.random.random(len(sol)) * scale - scale/2)
     return new
 
diff --git a/snp.py b/snp.py
index 3c2d438..2011f61 100644
--- a/snp.py
+++ b/snp.py
@@ -1,3 +1,4 @@
+#encoding: utf-8
 import math
 import numpy as np
 import matplotlib.pyplot as plt
@@ -44,6 +45,8 @@ if __name__=="__main__":
     can.add_argument("-e", "--steady-epsilon", metavar="DVAL", default=0, type=float,
             help="Stop if the improvement of the objective function value is lesser than DVAL")
 
+    can.add_argument("-a", "--variation-scale", metavar="RATIO", default=0.3, type=float,
+            help="Scale of the variation operators (as a ration of the domain width)")
 
     the = can.parse_args()
 
@@ -95,7 +98,7 @@ if __name__=="__main__":
                     dim = d * the.nb_sensors,
                     scale = the.domain_width),
                 make.neig(num.neighb_square,
-                    scale = the.domain_width/10,
+                    scale = the.variation_scale,
                     domain_width = the.domain_width),
                 iters
             )
@@ -110,7 +113,7 @@ if __name__=="__main__":
                     domain_width = the.domain_width,
                     nb_sensors = the.nb_sensors),
                 make.neig(bit.neighb_square,
-                    scale = the.domain_width/10,
+                    scale = the.variation_scale,
                     domain_width = the.domain_width),
                 iters
             )