What it means
A mortgage-backed security receives cash from a pool of loans, so when borrowers stop meeting their obligations, expected investor payments can change. CDR helps express new defaults over a comparable period; it is not simply the total share of borrowers who have ever missed a payment.
The SEC-hosted filing text defines a constant default rate as an annualised rate of default on a mortgage group used as an input to fair-value measurement. The exact contractual or model definition of default matters, as delinquency of a stated duration, foreclosure or another event may trigger classification.
A borrower behind on one instalment is not necessarily a realised loss, since cure, modification, collateral sale and recovery can change the amount ultimately lost. The Investopedia account describes a 90-day threshold as typical, but it is not a universal definition across every servicer, jurisdiction and securitisation.
Analysts often start with the nondefaulted pool balance at the beginning of a month and divide newly defaulted principal during that month by that base. For monthly default fraction d, one standard expression is 1 minus (1 minus d) raised to the twelfth power, which converts to an annual equivalent under a constant monthly hazard.
Annualising assumes a monthly pattern for comparison, not that the same homes default twelve times, and loans that default leave the performing denominator. Servicer reporting can lag economic distress, so a sudden rise in missed payments may affect future CDR even before loans meet a stated default definition.
Prepayments shrink the loan pool when borrowers repay early; they are not defaults, though they affect balances, cash flows and the denominator used in later months. CDR and constant prepayment rate answer different questions, and a security investor may need both to project coupon and principal timing.
Loss severity is the fraction of defaulted principal not recovered, and the total credit cost depends on default incidence and recovery, among other terms. A pool with many defaults but strong recoveries could produce a different loss than one with fewer defaults and weak collateral, so compare like-for-like assumptions.
Loan characteristics influence likely defaults, as borrower credit, property value, payment reset and local employment can all matter. Geographic concentration creates correlated risk, because a regional downturn may move many borrowers into difficulty at once.
A historical average CDR can be misleading when loan ages, underwriting or economic conditions change, so stress tests should use alternative paths. Investor cash flow also depends on tranche position and credit enhancement, so two securities backed by the same pool may bear very different default consequences; use the security's disclosure and model documentation to identify the actual definition, since the abbreviation alone cannot reconcile two reported default series.
In practice
Real-world examples.
Example
An MBS analyst compares annualised new defaults across two pools after matching their default definitions. One pool counts loans at 90 days past due, the other at 120 days, so the analyst restates the first on the second's basis before drawing conclusions. Without that step the comparison would be misleading.
Example
A servicer records rising 30-day delinquencies but does not yet classify those balances as new defaults. The monthly report shows $3 million of newly delinquent loans, yet new defaults stay at zero for the month. The investor team tracks the delinquent balance as an early-warning signal for the following months.
Example
A researcher stresses a pool with higher default rates and lower recovery assumptions to test tranche payments. She runs a baseline and two stress paths, then records at which scenario the lowest-ranking tranche begins to lose principal. The output informs how much credit enhancement looks adequate.
Formula
Calculation
Illustrative CDR = 1 - (1 - monthly new defaults / beginning nondefaulted balance)^12. If $1 million newly defaults from a $100 million performing pool, monthly d is 1% and annualised CDR is about 11.36%. This assumes a consistent monthly rate for comparison, not $12 million of certain annual defaults.
A second worked example uses a lower rate. If monthly d is 0.5%, then (1 - 0.005)^12 = 0.995^12, which is about 0.9416. CDR = 1 - 0.9416 = 0.0584, or about 5.84%. Halving the monthly rate roughly halves the annual rate, with the small difference arising because the shrinking pool compounds.Case study
Seen in the real world.
Fictional case: A fund studies a $200 million mortgage pool. In one month, $2 million of loans meet the transaction's stated default definition, while $5 million prepays early. The analyst calculates a 1% monthly new-default fraction using the appropriate nondefaulted starting balance, annualises it for a scenario and separately models prepayments. She then tests recovery values and each tranche's credit protection.
She does not call all late payments permanent losses, or use a high annualised CDR alone to estimate the exact amount the fund will lose. For the scenario, the 1% monthly fraction annualises to about 11.36%. She also assumes, purely for illustration, a loss severity of 40%, so the $2 million of defaulted principal implies a loss of about $800,000 before any credit enhancement absorbs it. Changing the severity assumption to 60% would raise that figure to $1.2 million, which is why she reports a range rather than a single number.
Watch out
Common mistakes.
- Treating delinquency, default and final realised loss as interchangeable labels.
- Including early repayments as defaulted loans or ignoring their effect on later balances.
- Comparing CDR figures without matching default definitions, denominators and periods.
Questions
People also ask.
Is CDR the same as cumulative default?
No. CDR expresses a period rate on an annualized basis; cumulative default tracks experience over time.
Does a 10% CDR mean a 10% investor loss?
No. Recoveries, timing and tranche protections affect actual losses.
Why annualize a monthly rate?
It offers a consistent comparison under a stated constant-rate assumption.
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