diff --git a/tests/difffloat.py b/tests/difffloat.py
index bbb7c95a1e77e04bbe21bec6dc6c5d529cd77c70..d4b48d54cbb9f292ed49b3cc142826cd1d71f87e 100644
--- a/tests/difffloat.py
+++ b/tests/difffloat.py
@@ -28,12 +28,18 @@ rel_tol = 1e-7
 # Comparisons are done both in absolute and relative terms
 
 # Individual tolerances for each column can be provided in a file
+# The (cube root of) the number of lines to check is provided as
+# an optional 4th argument
 
 file1 = sys.argv[1]
 file2 = sys.argv[2]
-fileTol = ""
+number_to_check = -1
+
+if len(sys.argv) == 5:
+    number_to_check = int(sys.argv[4])
 
-if len(sys.argv) == 4:
+fileTol = ""
+if len(sys.argv) >= 4:
     fileTol = sys.argv[3]
 
 data1 = loadtxt(file1)
@@ -67,8 +73,17 @@ else:
     absTol = dataTol[0,:]
     relTol = dataTol[1,:]
 
+n_lines_to_check = 0
+if number_to_check > 0:
+    n_lines_to_check = number_to_check**3
+    n_lines_to_check = min(n_lines_to_check, n_lines)
+    print "Checking the first %d particles."%n_lines_to_check
+else:
+    n_lines_to_check = n_lines
+    print "Checking all particles in the file."
+
 error = False
-for i in range(n_lines):
+for i in range(n_lines_to_check):
     for j in range(n_columns):
 
         abs_diff = abs(data1[i,j] - data2[i,j])
@@ -80,7 +95,7 @@ for i in range(n_lines):
             rel_diff = 0.
 
         if( abs_diff > absTol[j]):
-            print "Absolute difference larger than tolerance (%e) on line %d, column %d:"%(absTol[j], i,j)
+            print "Absolute difference larger than tolerance (%e) for particle %d, column %d:"%(absTol[j], i,j)
             print "%10s:           a = %e"%("File 1", data1[i,j])
             print "%10s:           b = %e"%("File 2", data2[i,j])
             print "%10s:       |a-b| = %e"%("Difference", abs_diff)
@@ -88,7 +103,7 @@ for i in range(n_lines):
             error = True
 
         if( rel_diff > relTol[j]):
-            print "Relative difference larger than tolerance (%e) on line %d, column %d:"%(relTol[j], i,j)
+            print "Relative difference larger than tolerance (%e) for particle %d, column %d:"%(relTol[j], i,j)
             print "%10s:           a = %e"%("File 1", data1[i,j])
             print "%10s:           b = %e"%("File 2", data2[i,j])
             print "%10s: |a-b|/|a+b| = %e"%("Difference", rel_diff)
diff --git a/tests/test27cells.sh b/tests/test27cells.sh
index 09d2513bd3ef404c7bf434948af7f10306c98ede..0afdc32c95397ce76190e847bdcd04a0b079ef78 100755
--- a/tests/test27cells.sh
+++ b/tests/test27cells.sh
@@ -3,6 +3,6 @@ rm brute_force_27_standard.dat swift_dopair_27_standard.dat
 
 ./test27cells -p 6 -r 1 -d 0 -f standard
 
-python difffloat.py brute_force_27_standard.dat swift_dopair_27_standard.dat tolerance.dat
+python difffloat.py brute_force_27_standard.dat swift_dopair_27_standard.dat tolerance.dat 6
 
 exit $?
diff --git a/tests/test27cellsPerturbed.sh b/tests/test27cellsPerturbed.sh
index 73d2933984d38f7dcc992f07ec2e016f3544b636..a553a2553e92cedee7c2c0679d231ec9d982fc28 100755
--- a/tests/test27cellsPerturbed.sh
+++ b/tests/test27cellsPerturbed.sh
@@ -3,6 +3,6 @@ rm brute_force_27_perturbed.dat swift_dopair_27_perturbed.dat
 
 ./test27cells -p 6 -r 1 -d 0.1 -f perturbed
 
-python difffloat.py brute_force_27_perturbed.dat swift_dopair_27_perturbed.dat tolerance.dat
+python difffloat.py brute_force_27_perturbed.dat swift_dopair_27_perturbed.dat tolerance.dat 6
 
 exit $?
diff --git a/tests/testPair.c b/tests/testPair.c
index 23ce4eb3de460f4e17b7b6f81cb39a628f3d100f..6e46b577ca63a8d3c2edce888a7485af0949813d 100644
--- a/tests/testPair.c
+++ b/tests/testPair.c
@@ -180,7 +180,7 @@ void runner_dopair1_density(struct runner *r, struct cell *ci, struct cell *cj);
 int main(int argc, char *argv[]) {
   size_t particles = 0, runs = 0, volume, type = 0;
   double offset[3] = {0, 0, 0}, h = 1.1255, size = 1., rho = 1.;
-  double perturbation = 0.1;
+  double perturbation = 0.;
   struct cell *ci, *cj;
   struct space space;
   struct engine engine;