diff --git a/lib/descriptive_statistics/class_methods.rb b/lib/descriptive_statistics/class_methods.rb
index 1988d1b..040fc1b 100644
--- a/lib/descriptive_statistics/class_methods.rb
+++ b/lib/descriptive_statistics/class_methods.rb
@@ -17,7 +17,7 @@ def empty_collection_default_value=(value)
       end
       define_method("#{m}_empty_collection_default_value=") do |value|
         default_values[m] = value
-      end      
+      end
     end
 
     DescriptiveStatistics.instance_methods.each do |m|
@@ -27,7 +27,7 @@ def empty_collection_default_value=(value)
     private
 
     def default_values
-      @default_values ||= {} 
+      @default_values ||= {}
     end
 
   end
diff --git a/lib/descriptive_statistics/mean.rb b/lib/descriptive_statistics/mean.rb
index cda8ffa..2d0076d 100644
--- a/lib/descriptive_statistics/mean.rb
+++ b/lib/descriptive_statistics/mean.rb
@@ -1,8 +1,8 @@
 module DescriptiveStatistics
   def mean(collection = self)
     values = Support::convert(collection)
-    return DescriptiveStatistics.mean_empty_collection_default_value unless values.size > 0
+    return DescriptiveStatistics.mean_empty_collection_default_value if values.empty?
 
     values.sum / values.number
-  end 
+  end
 end
diff --git a/lib/descriptive_statistics/median.rb b/lib/descriptive_statistics/median.rb
index 78657fe..8c9e8aa 100644
--- a/lib/descriptive_statistics/median.rb
+++ b/lib/descriptive_statistics/median.rb
@@ -1,8 +1,8 @@
 module DescriptiveStatistics
   def median(collection = self)
     values = Support::convert(collection)
-    return DescriptiveStatistics.median_empty_collection_default_value unless values.size > 0
+    return DescriptiveStatistics.median_empty_collection_default_value if values.empty?
 
     values.percentile(50)
-  end 
+  end
 end
diff --git a/lib/descriptive_statistics/mode.rb b/lib/descriptive_statistics/mode.rb
index ef6d09d..3b4fcc2 100644
--- a/lib/descriptive_statistics/mode.rb
+++ b/lib/descriptive_statistics/mode.rb
@@ -1,12 +1,12 @@
 module DescriptiveStatistics
   def mode(collection = self)
     values = Support::extract(collection)
-    return unless values.size > 0
+    return if values.to_a.empty?
 
     values
       .group_by { |e| e }
       .values
       .max_by(&:size)
       .first
-  end 
+  end
 end
diff --git a/lib/descriptive_statistics/percentile.rb b/lib/descriptive_statistics/percentile.rb
index 15d6926..776d590 100644
--- a/lib/descriptive_statistics/percentile.rb
+++ b/lib/descriptive_statistics/percentile.rb
@@ -1,17 +1,15 @@
 module DescriptiveStatistics
   def percentile(p, collection = self)
     values = Support::convert(collection)
-    return DescriptiveStatistics.percentile_empty_collection_default_value unless values.size > 0
 
-    return values.first unless values.size > 1
-
-    sorted = values.sort
-    return sorted[-1] if p == 100        
-    rank = p / 100.0 * (values.number - 1) 
-    lrank = rank.floor
-    d = rank - lrank
-    lower = sorted[lrank]
-    upper = sorted[lrank+1]
-    lower + (upper - lower) * d
+    if values.empty?; DescriptiveStatistics.percentile_empty_collection_default_value
+    elsif values.size == 1; values.first
+    else
+      values.sort!
+      return values.last if p == 100
+      rank = p / 100.0 * (values.size - 1)
+      lower, upper = values[rank.floor,2]
+      lower + (upper - lower) * (rank - rank.floor)
+    end
   end
 end
diff --git a/lib/descriptive_statistics/percentile_rank.rb b/lib/descriptive_statistics/percentile_rank.rb
index 674ea6f..6901777 100644
--- a/lib/descriptive_statistics/percentile_rank.rb
+++ b/lib/descriptive_statistics/percentile_rank.rb
@@ -2,8 +2,8 @@ module DescriptiveStatistics
   # percent of cases that are at or below a score
   def percentile_rank(p, collection = self)
     values = Support::convert(collection)
-    return DescriptiveStatistics.percentile_rank_empty_collection_default_value unless values.size > 0
+    return DescriptiveStatistics.percentile_rank_empty_collection_default_value if values.empty?
 
-    return (((values.sort.rindex{ |x| x <= p } || -1.0) + 1.0)) / values.number * 100.0
+    return (((values.sort.rindex { |x| x <= p } || -1.0) + 1.0)) / values.number * 100.0
   end
 end
diff --git a/lib/descriptive_statistics/range.rb b/lib/descriptive_statistics/range.rb
index 6f2c2ce..8189f2e 100644
--- a/lib/descriptive_statistics/range.rb
+++ b/lib/descriptive_statistics/range.rb
@@ -1,8 +1,8 @@
 module DescriptiveStatistics
   def range(collection = self)
     values = Support::convert(collection)
-    return DescriptiveStatistics.range_empty_collection_default_value unless values.size > 0
+    return DescriptiveStatistics.range_empty_collection_default_value if values.empty?
 
     values.max - values.min
-  end 
+  end
 end
diff --git a/lib/descriptive_statistics/standard_deviation.rb b/lib/descriptive_statistics/standard_deviation.rb
index f82e856..4e71654 100644
--- a/lib/descriptive_statistics/standard_deviation.rb
+++ b/lib/descriptive_statistics/standard_deviation.rb
@@ -1,7 +1,7 @@
 module DescriptiveStatistics
   def standard_deviation(collection = self)
     values = Support::convert(collection)
-    return DescriptiveStatistics.standard_deviation_empty_collection_default_value unless values.size > 0
+    return DescriptiveStatistics.standard_deviation_empty_collection_default_value if values.empty?
 
     Math.sqrt(values.variance)
   end
diff --git a/lib/descriptive_statistics/sum.rb b/lib/descriptive_statistics/sum.rb
index ff44be4..db068fb 100644
--- a/lib/descriptive_statistics/sum.rb
+++ b/lib/descriptive_statistics/sum.rb
@@ -1,8 +1,8 @@
 module DescriptiveStatistics
   def sum(collection = self)
     values = Support::convert(collection)
-    return DescriptiveStatistics.sum_empty_collection_default_value unless values.size > 0
+    return DescriptiveStatistics.sum_empty_collection_default_value if values.empty?
 
-    return values.inject(:+)
-  end 
+    return values.reduce(:+)
+  end
 end
diff --git a/lib/descriptive_statistics/support/convert.rb b/lib/descriptive_statistics/support/convert.rb
index 1b8da12..ee0486e 100644
--- a/lib/descriptive_statistics/support/convert.rb
+++ b/lib/descriptive_statistics/support/convert.rb
@@ -15,7 +15,7 @@ def self.extract(from_enumerable)
     private
 
     def self.extend(enumerable)
-      enumerable.extend(DescriptiveStatistics) 
+      enumerable.extend(DescriptiveStatistics)
     end
 
     def self.to_float(enumerable)
diff --git a/lib/descriptive_statistics/variance.rb b/lib/descriptive_statistics/variance.rb
index 257f2cd..ad3f7ab 100644
--- a/lib/descriptive_statistics/variance.rb
+++ b/lib/descriptive_statistics/variance.rb
@@ -1,9 +1,9 @@
 module DescriptiveStatistics
   def variance(collection = self)
     values = Support::convert(collection)
-    return DescriptiveStatistics.variance_empty_collection_default_value unless values.size > 0
+    return DescriptiveStatistics.variance_empty_collection_default_value if values.empty?
 
     mean = values.mean
-    values.map{ |sample| (mean - sample) ** 2 }.inject(:+) / values.number
+    values.map { |sample| (mean - sample) ** 2 }.reduce(:+) / values.number
   end
 end
diff --git a/spec/enumerable_spec.rb b/spec/enumerable_spec.rb
index 48ffebf..e1285a5 100644
--- a/spec/enumerable_spec.rb
+++ b/spec/enumerable_spec.rb
@@ -3,7 +3,7 @@
 describe "DescriptiveStatistics" do
   require 'descriptive_statistics'
 
-  class Foo 
+  class Foo
     include Enumerable
     attr_accessor :bar, :baz, :bat
 
diff --git a/test/test.rb b/test/test.rb
index d49617e..9cbcce1 100644
--- a/test/test.rb
+++ b/test/test.rb
@@ -3,7 +3,7 @@
 require './lib/descriptive_statistics'
 
 class TestData < MiniTest::Unit::TestCase
-  
+
   def setup
     @data = []
     CSV.foreach("test/testdata.csv") do |row|
@@ -14,31 +14,31 @@ def setup
   def test_sum
     @data.each do |test_case|
       assert_equal test_case[0,10].sum.round(6), test_case[10].round(6)
-    end  
+    end
   end
 
   def test_mean
     @data.each do |test_case|
       assert_equal test_case[0,10].mean.round(6), test_case[11].round(6)
-    end  
+    end
   end
-  
+
   def test_median
     @data.each do |test_case|
       assert_equal test_case[0,10].median.round(6), test_case[12].round(6)
-    end  
+    end
   end
 
   def test_variance
     @data.each do |test_case|
       assert_equal test_case[0,10].variance.round(6), test_case[13].round(6)
-    end  
+    end
   end
-  
+
   def test_standard_deviation
     @data.each do |test_case|
       assert_equal test_case[0,10].standard_deviation.round(6), test_case[14].round(6)
-    end  
+    end
   end
 
   def test_percentile
@@ -54,13 +54,13 @@ def test_percentile
       assert_equal test_case[0,10].percentile(80).round(6), test_case[23].round(6)
       assert_equal test_case[0,10].percentile(90).round(6), test_case[24].round(6)
       assert_equal test_case[0,10].percentile(100).round(6), test_case[25].round(6)
-    end  
+    end
   end
 
   def test_percentile_rank
     @data.each do |test_case|
       assert_equal test_case[0,10].percentile_rank(50).round(6), test_case[26].round(6)
-    end  
+    end
   end
 
   def test_percentile_rank_for_smallest_number
@@ -70,14 +70,14 @@ def test_percentile_rank_for_smallest_number
 
   def test_mode
     assert_equal [1,3,6,9,4,5,2,3,4,1,6,7,8,3,2,3,5,7,8,5,6,5,6,5,4,5,5,5].mode, 5
-  end    
+  end
 
   def test_range
     assert_equal [17, 5, 3, 23, 33, 30, 45, 37].range, 42
     assert_equal [].range, nil
     assert_equal [1, 1.0].range, 0
   end
-  
+
   def test_descriptive_statistics
     @data.each do |test_case|
       stat = test_case[0, 10].descriptive_statistics
@@ -95,7 +95,7 @@ def test_descriptive_statistics
       assert_equal test_case[0, 10].percentile(75), stat[:q3]
     end
   end
-  
+
   def test_median_is_percentile50
     @data.each do |test_case|
       assert_equal test_case[0, 10].median, test_case[0, 10].percentile(50)