diff --git a/container/statssum.go b/container/statssum.go
index 6126b877..cda8054b 100644
--- a/container/statssum.go
+++ b/container/statssum.go
@@ -26,6 +26,7 @@ import (
 type statsSummaryContainerHandlerWrapper struct {
 	handler          ContainerHandler
 	currentSummary   *info.ContainerStatsSummary
+	prevStats        *info.ContainerStats
 	totalMemoryUsage *big.Int
 	numStats         uint64
 	sampler          sampling.Sampler
diff --git a/info/container.go b/info/container.go
index 6dfba908..489a3506 100644
--- a/info/container.go
+++ b/info/container.go
@@ -15,6 +15,7 @@
 package info
 
 import (
+	"fmt"
 	"sort"
 	"time"
 )
@@ -186,17 +187,17 @@ type ContainerStatsSummary struct {
 // cumulative.
 // prev should be an earlier observation than current.
 // This method is not thread/goroutine safe.
-func (self *ContainerStatsSummary) AddSample(prev, current *ContainerStats) {
+func NewSample(prev, current *ContainerStats) (*ContainerStatsSample, error) {
 	if prev == nil || current == nil {
-		return
+		return nil, fmt.Errorf("empty stats")
 	}
 	// Ignore this sample if it is incomplete
 	if prev.Cpu == nil || prev.Memory == nil || current.Cpu == nil || current.Memory == nil {
-		return
+		return nil, fmt.Errorf("incomplete stats")
 	}
 	// prev must be an early observation
 	if !current.Timestamp.After(prev.Timestamp) {
-		return
+		return nil, fmt.Errorf("wrong stats order")
 	}
 	sample := new(ContainerStatsSample)
 	// Caculate the diff to get the CPU usage within the time interval.
@@ -204,8 +205,7 @@ func (self *ContainerStatsSummary) AddSample(prev, current *ContainerStats) {
 	// Memory usage is current memory usage
 	sample.Memory.Usage = current.Memory.Usage
 
-	self.Samples = append(self.Samples, sample)
-	return
+	return sample, nil
 }
 
 type uint64Slice []uint64
diff --git a/info/container_test.go b/info/container_test.go
index c1d1029c..50223403 100644
--- a/info/container_test.go
+++ b/info/container_test.go
@@ -85,9 +85,7 @@ func TestPercentiles(t *testing.T) {
 	}
 }
 
-func TestAddSampleNilStats(t *testing.T) {
-	s := &ContainerStatsSummary{}
-
+func TestNewSampleNilStats(t *testing.T) {
 	stats := &ContainerStats{
 		Cpu:    &CpuStats{},
 		Memory: &MemoryStats{},
@@ -97,23 +95,19 @@ func TestAddSampleNilStats(t *testing.T) {
 	stats.Cpu.Usage.System = uint64(2)
 	stats.Cpu.Usage.User = uint64(8)
 	stats.Memory.Usage = uint64(200)
-	s.AddSample(nil, stats)
 
-	if len(s.Samples) != 0 {
-		t.Errorf("added an unexpected sample: %+v", s.Samples)
+	sample, err := NewSample(nil, stats)
+	if err == nil {
+		t.Errorf("generated an unexpected sample: %+v", sample)
 	}
 
-	s.Samples = nil
-	s.AddSample(stats, nil)
-
-	if len(s.Samples) != 0 {
-		t.Errorf("added an unexpected sample: %+v", s.Samples)
+	sample, err = NewSample(nil, stats)
+	if err == nil {
+		t.Errorf("generated an unexpected sample: %+v", sample)
 	}
 }
 
 func TestAddSample(t *testing.T) {
-	s := &ContainerStatsSummary{}
-
 	cpuPrevUsage := uint64(10)
 	cpuCurrentUsage := uint64(15)
 	memCurrentUsage := uint64(200)
@@ -139,24 +133,24 @@ func TestAddSample(t *testing.T) {
 	current.Memory.Usage = memCurrentUsage
 	current.Timestamp = prev.Timestamp.Add(1 * time.Second)
 
-	s.AddSample(prev, current)
-
-	if len(s.Samples) != 1 {
-		t.Fatalf("unexpected samples: %+v", s.Samples)
+	sample, err := NewSample(prev, current)
+	if err != nil {
+		t.Errorf("should be able to generate a sample. but received error: %v", err)
+	}
+	if sample == nil {
+		t.Fatalf("nil sample and nil error. unexpected result!")
 	}
 
-	if s.Samples[0].Memory.Usage != memCurrentUsage {
-		t.Errorf("wrong memory usage: %v. should be %v", s.Samples[0].Memory.Usage, memCurrentUsage)
+	if sample.Memory.Usage != memCurrentUsage {
+		t.Errorf("wrong memory usage: %v. should be %v", sample.Memory.Usage, memCurrentUsage)
 	}
 
-	if s.Samples[0].Cpu.Usage != cpuCurrentUsage-cpuPrevUsage {
-		t.Errorf("wrong CPU usage: %v. should be %v", s.Samples[0].Cpu.Usage, cpuCurrentUsage-cpuPrevUsage)
+	if sample.Cpu.Usage != cpuCurrentUsage-cpuPrevUsage {
+		t.Errorf("wrong CPU usage: %v. should be %v", sample.Cpu.Usage, cpuCurrentUsage-cpuPrevUsage)
 	}
 }
 
 func TestAddSampleIncompleteStats(t *testing.T) {
-	s := &ContainerStatsSummary{}
-
 	cpuPrevUsage := uint64(10)
 	cpuCurrentUsage := uint64(15)
 	memCurrentUsage := uint64(200)
@@ -186,38 +180,30 @@ func TestAddSampleIncompleteStats(t *testing.T) {
 		Cpu:    prev.Cpu,
 		Memory: nil,
 	}
-	s.AddSample(stats, current)
-	if len(s.Samples) != 0 {
-		t.Errorf("added an unexpected sample: %+v", s.Samples)
+	sample, err := NewSample(stats, current)
+	if err == nil {
+		t.Errorf("generated an unexpected sample: %+v", sample)
 	}
-	s.Samples = nil
-
-	s.AddSample(prev, stats)
-	if len(s.Samples) != 0 {
-		t.Errorf("added an unexpected sample: %+v", s.Samples)
+	sample, err = NewSample(prev, stats)
+	if err == nil {
+		t.Errorf("generated an unexpected sample: %+v", sample)
 	}
-	s.Samples = nil
 
 	stats = &ContainerStats{
 		Cpu:    nil,
 		Memory: prev.Memory,
 	}
-	s.AddSample(stats, current)
-	if len(s.Samples) != 0 {
-		t.Errorf("added an unexpected sample: %+v", s.Samples)
+	sample, err = NewSample(stats, current)
+	if err == nil {
+		t.Errorf("generated an unexpected sample: %+v", sample)
 	}
-	s.Samples = nil
-
-	s.AddSample(prev, stats)
-	if len(s.Samples) != 0 {
-		t.Errorf("added an unexpected sample: %+v", s.Samples)
+	sample, err = NewSample(prev, stats)
+	if err == nil {
+		t.Errorf("generated an unexpected sample: %+v", sample)
 	}
-	s.Samples = nil
 }
 
 func TestAddSampleWrongOrder(t *testing.T) {
-	s := &ContainerStatsSummary{}
-
 	cpuPrevUsage := uint64(10)
 	cpuCurrentUsage := uint64(15)
 	memCurrentUsage := uint64(200)
@@ -243,8 +229,8 @@ func TestAddSampleWrongOrder(t *testing.T) {
 	current.Memory.Usage = memCurrentUsage
 	current.Timestamp = prev.Timestamp.Add(1 * time.Second)
 
-	s.AddSample(current, prev)
-	if len(s.Samples) != 0 {
-		t.Errorf("added an unexpected sample: %+v", s.Samples)
+	sample, err := NewSample(current, prev)
+	if err == nil {
+		t.Errorf("generated an unexpected sample: %+v", sample)
 	}
 }