diff --git a/ana/predictor.go b/ana/predictor.go
index 93ecbca..7f2aed7 100644
--- a/ana/predictor.go
+++ b/ana/predictor.go
@@ -38,18 +38,33 @@ func NewInterestPredictor(namePattern string, currencyPattern string, apy float6
 }
 
 func NewContributionPredictor(reg ledger.Register) Predictor {
-	monthlyRate := getMonthlyContributionRate(reg)
+	monthlyRate := getMonthlyContributionRates(reg)
 	_ = monthlyRate
 	return func(given ledger.Balances, delta time.Duration) ledger.Balances {
 		panic(nil)
 	}
 }
 
-func getMonthlyContributionRate(reg ledger.Register) map[string]ledger.Balance {
+func getMonthlyContributionRates(reg ledger.Register) map[string]ledger.Balance {
 	contributions := getRecentContributions(reg)
 	result := make(map[string]ledger.Balance)
 	for name := range contributions {
-		panic(nil)
+		result[name] = getMonthlyContributionRate(contributions[name])
+	}
+	return result
+}
+
+// TODO better than average
+func getMonthlyContributionRate(contributions []ledger.Balance) ledger.Balance {
+	sumPerCurrency := map[ledger.Currency]float64{}
+	for _, balance := range contributions {
+		for currency, v := range balance {
+			sumPerCurrency[currency] += v
+		}
+	}
+	result := make(ledger.Balance)
+	for currency, summed := range sumPerCurrency {
+		result[currency] = summed / float64(len(contributions))
 	}
 	return result
 }
@@ -62,10 +77,20 @@ func getContributions(reg ledger.Register) map[string][]ledger.Balance {
 	contributions := make(map[string][]ledger.Balance)
 	for _, date := range reg.Dates() {
 		for name := range reg[date] {
-			contributions[name] = append(contributions[name], maps.Clone(reg[date][name]))
-			if a := contributions[name]; len(a) > 1 {
-				for k := range a[len(a)-2] {
-					a[len(a)-1][k] -= a[len(a)-2][k]
+			contributions[name] = append(contributions[name], make(ledger.Balance))
+			if len(contributions[name]) > 1 {
+				for k := range contributions[name][len(contributions[name])-2] {
+					contributions[name][len(contributions[name])-1][k] = 0
+				}
+			}
+			balance := contributions[name][len(contributions[name])-1]
+			for currency, value := range reg[date][name] {
+				balance[currency] = value
+			}
+			if forName := contributions[name]; len(forName) > 1 {
+				lastBalance := forName[len(forName)-2]
+				for currency := range lastBalance {
+					balance[currency] -= lastBalance[currency]
 				}
 			}
 		}
diff --git a/ana/predictor_test.go b/ana/predictor_test.go
index bf8d486..77e7429 100644
--- a/ana/predictor_test.go
+++ b/ana/predictor_test.go
@@ -52,11 +52,12 @@ func TestGetContributions(t *testing.T) {
 			"a": ledger.Balance{"y": 1},
 		},
 		"2001-03": ledger.Balances{
-			"b": ledger.Balance{"z": 2},
+			"b": ledger.Balance{"z": 4},
 		},
 	}
 
 	got := getContributions(input)
+	t.Logf("%+v", got)
 	if len(got) != 2 {
 		t.Error(len(got))
 	}
@@ -69,9 +70,9 @@ func TestGetContributions(t *testing.T) {
 	} else if len(got["a"][1]) != 2 {
 		t.Error(got["a"][1])
 	} else if got["a"][1]["x"] != -1 {
-		t.Error(got["a"][1]["x"])
+		t.Error(got["a"][1])
 	} else if got["a"][1]["y"] != 1 {
-		t.Error(got["a"][1]["y"])
+		t.Error(got["a"][1])
 	}
 
 	if len(got["b"]) != 2 {
@@ -81,8 +82,26 @@ func TestGetContributions(t *testing.T) {
 	} else if got["b"][0]["z"] != 1 {
 		t.Error(got["b"][0])
 	} else if len(got["b"][1]) != 1 {
+		t.Error(got["b"])
+	} else if got["b"][1]["z"] != 3 {
 		t.Error(got["b"][1])
-	} else if got["b"][1]["z"] != 1 {
-		t.Error(got["b"][1]["z"])
+	}
+}
+
+func TestGetMonthlyContributionRate(t *testing.T) {
+	input := []ledger.Balance{
+		ledger.Balance{"x": 2},
+		ledger.Balance{"x": 4},
+		ledger.Balance{"y": 3},
+	}
+	got := getMonthlyContributionRate(input)
+	if len(got) != 2 {
+		t.Error(got)
+	}
+	if got["x"] != 2 {
+		t.Error(got["x"])
+	}
+	if got["y"] != 1 {
+		t.Error(got["y"])
 	}
 }