diff --git a/utilities/cross_validation.go b/cross_validation/cross_validation.go
similarity index 65%
rename from utilities/cross_validation.go
rename to cross_validation/cross_validation.go
index 25a7446..58e3acf 100644
--- a/utilities/cross_validation.go
+++ b/cross_validation/cross_validation.go
@@ -1,20 +1,24 @@
-package utilities
+package cross_validation
 
 import (
 	"fmt"
 	mat "github.com/gonum/matrix/mat64"
 	"math/rand"
+	"sync"
 	"time"
 )
 
-func shuffleMatrix(dataset *mat.Dense, numGen *rand.Rand) *mat.Dense {
+func shuffleMatrix(returnDatasets []*mat.Dense, dataset mat.Matrix, testSize int, seed int64, wg *sync.WaitGroup) {
+	numGen := rand.New(rand.NewSource(seed))
+
+	// We don't want to alter the original dataset.
 	shuffledSet := mat.DenseCopyOf(dataset)
 	rowCount, colCount := shuffledSet.Dims()
 	temp := make([]float64, colCount)
-
+	
 	// Fisher–Yates shuffle
 	for i := 0; i < rowCount; i++ {
-		j := numGen.Intn(i + 1)
+		j := numGen.Intn(i+1)
 		if j != i {
 			// Make a "hard" copy to avoid pointer craziness.
 			copy(temp, shuffledSet.RowView(i))
@@ -22,8 +26,11 @@ func shuffleMatrix(dataset *mat.Dense, numGen *rand.Rand) *mat.Dense {
 			shuffledSet.SetRow(j, temp)
 		}
 	}
+	trainSize := rowCount - testSize
+	returnDatasets[0] = mat.NewDense(trainSize, colCount, shuffledSet.RawMatrix().Data[:trainSize*colCount])
+	returnDatasets[1] = mat.NewDense(testSize, colCount, shuffledSet.RawMatrix().Data[trainSize*colCount:])
 
-	return shuffledSet
+	wg.Done()
 }
 
 // TrainTestSplit splits input DenseMatrix into subsets for testing.
@@ -49,41 +56,41 @@ func TrainTestSplit(size interface{}, randomState interface{}, datasets ...*mat.
 		}
 	}
 
-	var trainSize, testSize int
+	var testSize int
 	switch size := size.(type) {
 		// If size is an integer, treat it as the test data instance count.
 	case int:
-		trainSize = instanceCount - size
 		testSize = size
 	case float64:
 		// If size is a float, treat it as a percentage of the instances to be allocated to the test set.
-		trainSize = int(float64(instanceCount)*(1-size) + 0.5)
 		testSize = int(float64(instanceCount)*size + 0.5)
 	default:
 		return nil, fmt.Errorf("Expected a test instance count (int) or percentage (float64)")
 	}
 
+	var randSeed int64
 	// Create a deterministic shuffle, or a "random" one based on current time.
-	var randSource rand.Source
 	if seed, ok := randomState.(int); ok {
-		randSource = rand.NewSource(int64(seed))
+		randSeed = int64(seed)
 	} else {
-		randSource = rand.NewSource(time.Now().Unix())
+		// Use seconds since epoch as seed
+		randSeed = time.Now().Unix()
 	}
-	numGen := rand.New(randSource)
+
+	// Wait group for goroutine syncronization.
+	wg := new(sync.WaitGroup)
+	wg.Add(dataCount)
 
 	// Return slice will hold training and test data and optional labels matrix.
-	var returnDatasets []*mat.Dense
+	returnDatasets := make([]*mat.Dense, 2*dataCount)
 
-	for _, dataset := range datasets {
-		_, featureCount := dataset.Dims()
-
-		tempMatrix := shuffleMatrix(dataset, numGen)
-
-		// Features count is different on data and labels.
-		returnDatasets = append(returnDatasets, mat.NewDense(trainSize, featureCount, tempMatrix.RawMatrix().Data[:trainSize*featureCount]))
-		returnDatasets = append(returnDatasets, mat.NewDense(testSize, featureCount, tempMatrix.RawMatrix().Data[trainSize*featureCount:]))
+	for i, dataset := range datasets {
+		// Send proper returnDataset slice.
+		// This is needed so goroutine doesn't mess up the expected return order.
+		// Perhaps returning a map is a better solution...
+		go shuffleMatrix(returnDatasets[i:i+2], dataset, testSize, randSeed, wg)
 	}
+	wg.Wait()
 
 	return returnDatasets, nil
 }
diff --git a/cross_validation/cross_validation_test.go b/cross_validation/cross_validation_test.go
new file mode 100644
index 0000000..5340c37
--- /dev/null
+++ b/cross_validation/cross_validation_test.go
@@ -0,0 +1,61 @@
+package cross_validation
+
+import (
+	//. "github.com/smartystreets/goconvey/convey"
+	mat "github.com/gonum/matrix/mat64"
+	"math/rand"
+	"testing"
+	"time"
+)
+
+var (
+	flatValues, flatLabels []float64
+	values, labels         *mat.Dense
+)
+
+func init() {
+	flatValues = make([]float64, 80)
+	flatLabels = make([]float64, 20)
+
+	for i := 0; i < 80; i++ {
+		flatValues[i] = float64(i + 1)
+		// Replaces labels four times per run but who cares.
+		flatLabels[int(i/4)] = float64(rand.Intn(2))
+	}
+
+	values = mat.NewDense(20, 4, flatValues)
+	labels = mat.NewDense(20, 1, flatLabels)
+}
+
+func TestTrainTrainTestSplit(t *testing.T) {
+	nolab1, err := TrainTestSplit(4, nil, values)
+	if err != nil {
+		t.Error(err)
+	}
+
+	// Make sure the random generator gets a new seed (time).
+	time.Sleep(time.Second)
+
+	nolab2, _ := TrainTestSplit(4, nil, values)
+	if nolab1[0].Equals(nolab2[0]) {
+		t.Errorf("Shuffle with different seed returned same matrix")
+	}
+
+	nolab1, _ = TrainTestSplit(4, 1, values)
+	nolab2, _ = TrainTestSplit(4, 1, values)
+	// Comparing the determinants does not guarantee uniqueness, but it will do for now.
+	if !nolab1[0].Equals(nolab2[0]) {
+		t.Errorf("Shuffle with same seed returned different matrix")
+	}
+
+	// Same thing for data with labels.
+	lab1, err := TrainTestSplit(0.1, 10, values, labels)
+	if err != nil {
+		t.Error(err)
+	}
+
+	lab2, _ := TrainTestSplit(0.1, 10, values, labels)
+	if !lab1[0].Equals(lab2[0]) {
+		t.Errorf("Shuffle with same seed returned different determinants")
+	}
+}