diff --git a/attr.go b/attr.go
index 445d821..55a22d4 100644
--- a/attr.go
+++ b/attr.go
@@ -40,10 +40,18 @@ func (a Attrs) SetTitle(title string) Attrs {
 }
 
 func (a Attrs) SetPert(opt, real, pess float64) Attrs {
-	a["pert"] = PertAttrs{
+	a["pert"] = &PertAttrs{
 		Optimistic:  opt,
 		Realistic:   real,
 		Pessimistic: pess,
 	}
 	return a
 }
+
+func (a Attrs) GetPert() *PertAttrs {
+	pa, found := a["pert"]
+	if !found {
+		return nil
+	}
+	return pa.(*PertAttrs)
+}
diff --git a/edge.go b/edge.go
index f0646c0..5436658 100644
--- a/edge.go
+++ b/edge.go
@@ -11,10 +11,14 @@ type Edge struct {
 	Attrs
 }
 
+type EdgeCostFN func(e *Edge) int64
+
 func newEdge(src, dst *Vertex, attrs ...Attrs) *Edge {
 	var a Attrs
 	if len(attrs) > 0 {
 		a = attrs[0]
+	} else {
+		a = make(map[string]interface{})
 	}
 	return &Edge{
 		src:   src,
diff --git a/graph.go b/graph.go
index 75b8741..9ace9b5 100644
--- a/graph.go
+++ b/graph.go
@@ -115,6 +115,11 @@ func (g Graph) findAllPathsRec(current, target *Vertex, prefix Path) Paths {
 }
 
 func (g Graph) FindShortestPath(srcID, dstID string) (Path, int64) {
+	costFN := func(e *Edge) int64 { return 1 }
+	return g.FindShortestPathFN(srcID, dstID, costFN)
+}
+
+func (g Graph) FindShortestPathFN(srcID, dstID string, fn EdgeCostFN) (Path, int64) {
 	src := g.GetVertex(srcID)
 	dst := g.GetVertex(dstID)
 	if src == nil || dst == nil {
@@ -147,7 +152,7 @@ func (g Graph) FindShortestPath(srcID, dstID string) (Path, int64) {
 			if n.dijkstra.visited {
 				continue
 			}
-			dist := u.dijkstra.dist + 1 // 1 could be replace by a value
+			dist := u.dijkstra.dist + fn(e)
 			if dist < n.dijkstra.dist {
 				n.dijkstra.dist = dist
 				n.dijkstra.prev = e
diff --git a/pert.go b/pert.go
index 56d05bd..74ba945 100644
--- a/pert.go
+++ b/pert.go
@@ -2,6 +2,7 @@ package graphman
 
 import (
 	"fmt"
+	"math"
 	"strconv"
 	"strings"
 	"time"
@@ -16,7 +17,7 @@ type PertAttrs struct {
 	Pessimistic, Realistic, Optimistic float64
 }
 
-func (pa PertAttrs) Estimate() float64 {
+func (pa PertAttrs) WeightedEstimate() float64 {
 	return (pa.Pessimistic + pa.Optimistic + 4*pa.Realistic) / 6
 }
 
@@ -29,13 +30,17 @@ func (pa PertAttrs) Variance() float64 {
 	return sd * sd
 }
 
+func (pa PertAttrs) AverageEstimate() float64 {
+	return (pa.Pessimistic + pa.Optimistic + pa.Realistic) / 3
+}
+
 func (pa PertAttrs) String() string {
 	return fmt.Sprintf(
 		"To=%s,Tm=%s,Tp=%s,Te=%s,σe=%s,Ve=%s",
 		prettyFloat(pa.Optimistic),
 		prettyFloat(pa.Realistic),
 		prettyFloat(pa.Pessimistic),
-		prettyFloat(pa.Estimate()),
+		prettyFloat(pa.WeightedEstimate()),
 		prettyFloat(pa.StandardDeviation()),
 		prettyFloat(pa.Variance()),
 	)
@@ -47,3 +52,43 @@ func prettyFloat(f float64) string {
 	out = strings.TrimRight(out, ".")
 	return out
 }
+
+type PertResult struct {
+	CriticalPathVariance          float64
+	CriticalPathStandardDeviation float64
+	CriticalPath                  Path
+}
+
+func (pr PertResult) String() string {
+	return fmt.Sprintf(
+		"Tσe=%s,TVe=%s",
+		prettyFloat(pr.CriticalPathStandardDeviation),
+		prettyFloat(pr.CriticalPathVariance),
+	)
+}
+
+func ComputePert(g *Graph) PertResult {
+	result := PertResult{}
+
+	// apply pert defaults before computing
+	for _, edge := range g.edges {
+		pa := edge.GetPert()
+		if pa == nil {
+			if edge.Attrs == nil {
+				edge.Attrs = make(map[string]interface{})
+			}
+			edge.SetPert(1, 1, 1)
+			pa = edge.GetPert()
+		}
+		if pa.Realistic == 0 {
+			pa.Realistic = 1
+		}
+		if pa.Pessimistic == 0 && pa.Optimistic == 0 {
+			pa.Pessimistic = pa.Realistic
+			pa.Optimistic = pa.Realistic
+		}
+		result.CriticalPathVariance += pa.Variance()
+	}
+	result.CriticalPathStandardDeviation = math.Sqrt(result.CriticalPathVariance)
+	return result
+}
diff --git a/pert_test.go b/pert_test.go
index 1468530..3037075 100644
--- a/pert_test.go
+++ b/pert_test.go
@@ -4,28 +4,102 @@ import (
 	"fmt"
 )
 
-func Example_pert() {
+func ExamplePertResult_withValue() {
 	graph := New()
-	graph.AddEdge("1", "2", Attrs{}.SetTitle("a").SetPert(3, 6, 15))
-	graph.AddEdge("1", "3", Attrs{}.SetTitle("b").SetPert(2, 5, 14))
-	graph.AddEdge("1", "4", Attrs{}.SetTitle("c").SetPert(6, 12, 30))
-	graph.AddEdge("2", "5", Attrs{}.SetTitle("d").SetPert(2, 5, 8))
-	graph.AddEdge("2", "6", Attrs{}.SetTitle("e").SetPert(5, 11, 17))
-	graph.AddEdge("3", "6", Attrs{}.SetTitle("f").SetPert(3, 6, 15))
-	graph.AddEdge("4", "7", Attrs{}.SetTitle("g").SetPert(3, 9, 27))
-	graph.AddEdge("5", "7", Attrs{}.SetTitle("h").SetPert(1, 4, 7))
-	graph.AddEdge("6", "7", Attrs{}.SetTitle("i").SetPert(4, 19, 28))
+	graph.AddEdge("1", "2", Attrs{}.SetPert(3, 6, 15))
+	graph.AddEdge("1", "3", Attrs{}.SetPert(2, 5, 14))
+	graph.AddEdge("1", "4", Attrs{}.SetPert(6, 12, 30))
+	graph.AddEdge("2", "5", Attrs{}.SetPert(2, 5, 8))
+	graph.AddEdge("2", "6", Attrs{}.SetPert(5, 11, 17))
+	graph.AddEdge("3", "6", Attrs{}.SetPert(3, 6, 15))
+	graph.AddEdge("4", "7", Attrs{}.SetPert(3, 9, 27))
+	graph.AddEdge("5", "7", Attrs{}.SetPert(1, 4, 7))
+	graph.AddEdge("6", "7", Attrs{}.SetPert(4, 19, 28))
+
+	fmt.Println("graph before computing:")
+	for _, e := range graph.Edges() {
+		fmt.Println("*", e)
+	}
+	fmt.Println()
+
+	result := ComputePert(graph)
+	fmt.Println("graph after computing:")
 	for _, e := range graph.Edges() {
 		fmt.Println("*", e)
 	}
+	fmt.Println("result:", result)
+
+	// Output:
+	// graph before computing:
+	// * (1,2)[[pert:To=3,Tm=6,Tp=15,Te=7,σe=2,Ve=4]]
+	// * (1,3)[[pert:To=2,Tm=5,Tp=14,Te=6,σe=2,Ve=4]]
+	// * (1,4)[[pert:To=6,Tm=12,Tp=30,Te=14,σe=4,Ve=16]]
+	// * (2,5)[[pert:To=2,Tm=5,Tp=8,Te=5,σe=1,Ve=1]]
+	// * (2,6)[[pert:To=5,Tm=11,Tp=17,Te=11,σe=2,Ve=4]]
+	// * (3,6)[[pert:To=3,Tm=6,Tp=15,Te=7,σe=2,Ve=4]]
+	// * (4,7)[[pert:To=3,Tm=9,Tp=27,Te=11,σe=4,Ve=16]]
+	// * (5,7)[[pert:To=1,Tm=4,Tp=7,Te=4,σe=1,Ve=1]]
+	// * (6,7)[[pert:To=4,Tm=19,Tp=28,Te=18,σe=4,Ve=16]]
+	//
+	// graph after computing:
+	// * (1,2)[[pert:To=3,Tm=6,Tp=15,Te=7,σe=2,Ve=4]]
+	// * (1,3)[[pert:To=2,Tm=5,Tp=14,Te=6,σe=2,Ve=4]]
+	// * (1,4)[[pert:To=6,Tm=12,Tp=30,Te=14,σe=4,Ve=16]]
+	// * (2,5)[[pert:To=2,Tm=5,Tp=8,Te=5,σe=1,Ve=1]]
+	// * (2,6)[[pert:To=5,Tm=11,Tp=17,Te=11,σe=2,Ve=4]]
+	// * (3,6)[[pert:To=3,Tm=6,Tp=15,Te=7,σe=2,Ve=4]]
+	// * (4,7)[[pert:To=3,Tm=9,Tp=27,Te=11,σe=4,Ve=16]]
+	// * (5,7)[[pert:To=1,Tm=4,Tp=7,Te=4,σe=1,Ve=1]]
+	// * (6,7)[[pert:To=4,Tm=19,Tp=28,Te=18,σe=4,Ve=16]]
+	// result: Tσe=8.12,TVe=66
+}
+
+func ExamplePertResult_withoutValue() {
+	graph := New()
+	graph.AddEdge("1", "2")
+	graph.AddEdge("1", "3")
+	graph.AddEdge("1", "4")
+	graph.AddEdge("2", "5")
+	graph.AddEdge("2", "6")
+	graph.AddEdge("3", "6")
+	graph.AddEdge("4", "7")
+	graph.AddEdge("5", "7")
+	graph.AddEdge("6", "7")
+
+	fmt.Println("graph before computing:")
+	for _, e := range graph.Edges() {
+		fmt.Println("*", e)
+	}
+	fmt.Println()
+
+	result := ComputePert(graph)
+	fmt.Println("graph after computing:")
+	for _, e := range graph.Edges() {
+		fmt.Println("*", e)
+	}
+	fmt.Println("result:", result)
+
 	// Output:
-	// * (1,2)[[pert:To=3,Tm=6,Tp=15,Te=7,σe=2,Ve=4,title:a]]
-	// * (1,3)[[pert:To=2,Tm=5,Tp=14,Te=6,σe=2,Ve=4,title:b]]
-	// * (1,4)[[pert:To=6,Tm=12,Tp=30,Te=14,σe=4,Ve=16,title:c]]
-	// * (2,5)[[pert:To=2,Tm=5,Tp=8,Te=5,σe=1,Ve=1,title:d]]
-	// * (2,6)[[pert:To=5,Tm=11,Tp=17,Te=11,σe=2,Ve=4,title:e]]
-	// * (3,6)[[pert:To=3,Tm=6,Tp=15,Te=7,σe=2,Ve=4,title:f]]
-	// * (4,7)[[pert:To=3,Tm=9,Tp=27,Te=11,σe=4,Ve=16,title:g]]
-	// * (5,7)[[pert:To=1,Tm=4,Tp=7,Te=4,σe=1,Ve=1,title:h]]
-	// * (6,7)[[pert:To=4,Tm=19,Tp=28,Te=18,σe=4,Ve=16,title:i]]
+	// graph before computing:
+	// * (1,2)
+	// * (1,3)
+	// * (1,4)
+	// * (2,5)
+	// * (2,6)
+	// * (3,6)
+	// * (4,7)
+	// * (5,7)
+	// * (6,7)
+	//
+	// graph after computing:
+	// * (1,2)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// * (1,3)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// * (1,4)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// * (2,5)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// * (2,6)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// * (3,6)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// * (4,7)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// * (5,7)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// * (6,7)[[pert:To=1,Tm=1,Tp=1,Te=1,σe=0,Ve=0]]
+	// result: Tσe=0,TVe=0
 }
diff --git a/vertex.go b/vertex.go
index 587452b..8f1a6ee 100644
--- a/vertex.go
+++ b/vertex.go
@@ -26,6 +26,8 @@ func newVertex(id string, attrs ...Attrs) *Vertex {
 	var a Attrs
 	if len(attrs) > 0 {
 		a = attrs[0]
+	} else {
+		a = make(map[string]interface{})
 	}
 	return &Vertex{
 		id:           id,