diff --git a/decor/eta.go b/decor/eta.go
index f0c97dd..6cb27a2 100644
--- a/decor/eta.go
+++ b/decor/eta.go
@@ -23,9 +23,9 @@
 }
 
 // EwmaETA exponential-weighted-moving-average based ETA decorator.
-// Note that it's necessary to supply bar.Incr* methods with incremental
-// work duration as second argument, in order for this decorator to
-// work correctly. This decorator is a wrapper of MovingAverageETA.
+// For this decorator to work correctly you have to measure each
+// iteration's duration and pass it to the
+// *Bar.DecoratorEwmaUpdate(time.Duration) method after each increment.
 func EwmaETA(style TimeStyle, age float64, wcc ...WC) Decorator {
 	var average ewma.MovingAverage
 	if age == 0 {
@@ -33,8 +33,7 @@
 	} else {
 		average = ewma.NewMovingAverage(age)
 	}
-	average = &ThreadSafeMovingAverage{MovingAverage: average}
-	return MovingAverageETA(style, average, nil, wcc...)
+	return MovingAverageETA(style, NewThreadSafeMovingAverage(average), nil, wcc...)
 }
 
 // MovingAverageETA decorator relies on MovingAverage implementation to calculate its average.
diff --git a/decor/moving_average.go b/decor/moving_average.go
index 5d2d3a0..50ac9c3 100644
--- a/decor/moving_average.go
+++ b/decor/moving_average.go
@@ -7,28 +7,36 @@
 	"github.com/VividCortex/ewma"
 )
 
-// ThreadSafeMovingAverage is a thread safe wrapper for ewma.MovingAverage.
-type ThreadSafeMovingAverage struct {
+type threadSafeMovingAverage struct {
 	ewma.MovingAverage
 	mu sync.Mutex
 }
 
-func (s *ThreadSafeMovingAverage) Add(value float64) {
+func (s *threadSafeMovingAverage) Add(value float64) {
 	s.mu.Lock()
 	s.MovingAverage.Add(value)
 	s.mu.Unlock()
 }
 
-func (s *ThreadSafeMovingAverage) Value() float64 {
+func (s *threadSafeMovingAverage) Value() float64 {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	return s.MovingAverage.Value()
 }
 
-func (s *ThreadSafeMovingAverage) Set(value float64) {
+func (s *threadSafeMovingAverage) Set(value float64) {
 	s.mu.Lock()
 	s.MovingAverage.Set(value)
 	s.mu.Unlock()
+}
+
+// NewThreadSafeMovingAverage converts provided ewma.MovingAverage
+// into thread safe ewma.MovingAverage.
+func NewThreadSafeMovingAverage(average ewma.MovingAverage) ewma.MovingAverage {
+	if tsma, ok := average.(*threadSafeMovingAverage); ok {
+		return tsma
+	}
+	return &threadSafeMovingAverage{MovingAverage: average}
 }
 
 type medianWindow [3]float64
@@ -56,5 +64,5 @@
 
 // NewMedian is fixed last 3 samples median MovingAverage.
 func NewMedian() ewma.MovingAverage {
-	return &ThreadSafeMovingAverage{MovingAverage: new(medianWindow)}
+	return NewThreadSafeMovingAverage(new(medianWindow))
 }
diff --git a/decor/speed.go b/decor/speed.go
index 142e5be..8a48e3f 100644
--- a/decor/speed.go
+++ b/decor/speed.go
@@ -28,9 +28,9 @@
 }
 
 // EwmaSpeed exponential-weighted-moving-average based speed decorator.
-// Note that it's necessary to supply bar.Incr* methods with incremental
-// work duration as second argument, in order for this decorator to
-// work correctly. This decorator is a wrapper of MovingAverageSpeed.
+// For this decorator to work correctly you have to measure each
+// iteration's duration and pass it to the
+// *Bar.DecoratorEwmaUpdate(time.Duration) method after each increment.
 func EwmaSpeed(unit int, format string, age float64, wcc ...WC) Decorator {
 	var average ewma.MovingAverage
 	if age == 0 {
@@ -38,8 +38,7 @@
 	} else {
 		average = ewma.NewMovingAverage(age)
 	}
-	average = &ThreadSafeMovingAverage{MovingAverage: average}
-	return MovingAverageSpeed(unit, format, average, wcc...)
+	return MovingAverageSpeed(unit, format, NewThreadSafeMovingAverage(average), wcc...)
 }
 
 // MovingAverageSpeed decorator relies on MovingAverage implementation