diff --git a/ana/predictor.go b/ana/predictor.go
index 7f2aed7..e5939de 100644
--- a/ana/predictor.go
+++ b/ana/predictor.go
@@ -46,16 +46,17 @@ func NewContributionPredictor(reg ledger.Register) Predictor {
 }
 
 func getMonthlyContributionRates(reg ledger.Register) map[string]ledger.Balance {
-	contributions := getRecentContributions(reg)
+	window := time.Hour * 24 * 365 / 2
+	contributions := getRecentContributions(reg, window)
 	result := make(map[string]ledger.Balance)
 	for name := range contributions {
-		result[name] = getMonthlyContributionRate(contributions[name])
+		result[name] = getMonthlyContributionRate(contributions[name], window)
 	}
 	return result
 }
 
 // TODO better than average
-func getMonthlyContributionRate(contributions []ledger.Balance) ledger.Balance {
+func getMonthlyContributionRate(contributions []ledger.Balance, window time.Duration) ledger.Balance {
 	sumPerCurrency := map[ledger.Currency]float64{}
 	for _, balance := range contributions {
 		for currency, v := range balance {
@@ -64,13 +65,13 @@ func getMonthlyContributionRate(contributions []ledger.Balance) ledger.Balance {
 	}
 	result := make(ledger.Balance)
 	for currency, summed := range sumPerCurrency {
-		result[currency] = summed / float64(len(contributions))
+		result[currency] = summed / (float64(window) / float64(time.Hour*24*365/12))
 	}
 	return result
 }
 
-func getRecentContributions(reg ledger.Register) map[string][]ledger.Balance {
-	return getContributions(reg.Between(time.Now().Add(-1*time.Hour*365/2), time.Now()))
+func getRecentContributions(reg ledger.Register, window time.Duration) map[string][]ledger.Balance {
+	return getContributions(reg.Between(time.Now().Add(-1*window), time.Now()))
 }
 
 func getContributions(reg ledger.Register) map[string][]ledger.Balance {
diff --git a/ana/predictor_test.go b/ana/predictor_test.go
index 77e7429..4bf4dc2 100644
--- a/ana/predictor_test.go
+++ b/ana/predictor_test.go
@@ -94,7 +94,7 @@ func TestGetMonthlyContributionRate(t *testing.T) {
 		ledger.Balance{"x": 4},
 		ledger.Balance{"y": 3},
 	}
-	got := getMonthlyContributionRate(input)
+	got := getMonthlyContributionRate(input, time.Hour*24*365/4)
 	if len(got) != 2 {
 		t.Error(got)
 	}