diff --git a/ana/prediction_test.go b/ana/prediction_test.go
index d3544c2..61a9db7 100644
--- a/ana/prediction_test.go
+++ b/ana/prediction_test.go
@@ -1,7 +1,6 @@
 package ana
 
 import (
-	"fmt"
 	"maps"
 	"testing"
 	"time"
@@ -31,8 +30,12 @@ func TestPredictionPredict(t *testing.T) {
 		return result
 	}
 	from := time.Now()
+	month := time.Hour * 24 * 365 / 12
 	these := []time.Time{time.Now().Add(time.Hour * 24 * 365 / 12)}
 	theseK := these[0].Format("2006-01")
+	thisMonth := time.Now().Add(-1 * time.Duration(time.Now().Day()) * time.Hour * 24)
+	monthsAgo1 := thisMonth.Add(-1 * month).Format("2006-01")
+	monthsAgo2 := thisMonth.Add(-2 * month).Format("2006-01")
 	_, _ = inc, double
 
 	cases := map[string]struct {
@@ -63,14 +66,55 @@ func TestPredictionPredict(t *testing.T) {
 			given:      ledger.Balances{"X": ledger.Balance{"X": 5}},
 			want:       ledger.Register{theseK: ledger.Balances{"X": ledger.Balance{"X": 11}}},
 		},
+		"contribution": {
+			prediction: Prediction{
+				NewContributionPredictor(ledger.Register{
+					"2001-01":  ledger.Balances{"X": ledger.Balance{"X": 100}},   // too old
+					"2001-02":  ledger.Balances{"X": ledger.Balance{"X": 10000}}, // too old
+					monthsAgo2: ledger.Balances{"X": ledger.Balance{"X": 100}},
+					monthsAgo1: ledger.Balances{"X": ledger.Balance{"X": 600}}, // +500 once in 6 months
+				}),
+			},
+			given: ledger.Balances{"X": ledger.Balance{"X": 5}},
+			want:  ledger.Register{theseK: ledger.Balances{"X": ledger.Balance{"X": 105}}},
+		},
+		"interest": {
+			prediction: Prediction{
+				NewInterestPredictor("X", "X", 12),
+			},
+			given: ledger.Balances{"X": ledger.Balance{"X": 5}},
+			want:  ledger.Register{theseK: ledger.Balances{"X": ledger.Balance{"X": 10}}},
+		},
+		"interest, contribution": {
+			prediction: Prediction{
+				NewInterestPredictor("X", "X", 12),
+				NewContributionPredictor(ledger.Register{
+					monthsAgo2: ledger.Balances{"X": ledger.Balance{"X": 100}},
+					monthsAgo1: ledger.Balances{"X": ledger.Balance{"X": 600}},
+				}),
+			},
+			given: ledger.Balances{"X": ledger.Balance{"X": 5}},
+			want:  ledger.Register{theseK: ledger.Balances{"X": ledger.Balance{"X": 110}}},
+		},
+		"contribution, interest": {
+			prediction: Prediction{
+				NewContributionPredictor(ledger.Register{
+					monthsAgo2: ledger.Balances{"X": ledger.Balance{"X": 100}},
+					monthsAgo1: ledger.Balances{"X": ledger.Balance{"X": 600}},
+				}),
+				NewInterestPredictor("X", "X", 12),
+			},
+			given: ledger.Balances{"X": ledger.Balance{"X": 5}},
+			want:  ledger.Register{theseK: ledger.Balances{"X": ledger.Balance{"X": 210}}},
+		},
 	}
 
 	for name, d := range cases {
 		c := d
 		t.Run(name, func(t *testing.T) {
 			got := c.prediction.predict(c.given, from, these)
-			if fmt.Sprintf("%+v", got) != fmt.Sprintf("%+v", c.want) {
-				t.Errorf("want\n\t%+v, got\n\t%+v", c.want, got)
+			if got.Debug() != c.want.Debug() {
+				t.Errorf("want\n\t%+v, got\n\t%+v", c.want.Debug(), got.Debug())
 			}
 		})
 	}
diff --git a/ledger/register.go b/ledger/register.go
index b223c7d..a72cec6 100644
--- a/ledger/register.go
+++ b/ledger/register.go
@@ -100,3 +100,19 @@ func dateToTime(s string) (time.Time, error) {
 	}
 	return time.Time{}, fmt.Errorf("no layout matching %q", s)
 }
+
+func (register Register) Debug() string {
+	result := "{"
+	for _, date := range register.Dates() {
+		result += "{" + date
+		for name, balance := range register[date] {
+			result += "{" + name
+			for cur, v := range balance {
+				result += fmt.Sprintf("%s=%.2f ", cur, v)
+			}
+			result += "}"
+		}
+		result += "}"
+	}
+	return result + "}"
+}