Single source

Glossary definition of "single source," the practice of buying a category from one vendor without competitive rebid, and how it shows margin drift.

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Single source

Single source is a purchasing arrangement where one vendor supplies an entire category or item with no competing bidder held in reserve. Margin drift is the gap between what a vendor contract says and what the invoice actually charges. Single source arrangements do not cause drift by themselves, but they remove the one check that normally catches it: a second vendor's invoice to compare against.

1. What is single source?

Single source is a sourcing decision to buy an entire category or part number from one vendor, with no alternate supplier qualified or invited to bid. It differs from sole source, where only one vendor exists in the market. A single source vendor is chosen, not forced, usually for switching cost, integration, or certification reasons, and the choice is a business decision, not a defect.

The term describes the buyer's decision, not the vendor's market position.

2. Why does single source increase margin drift risk?

A competing bid is a recurring, informal audit: it forces a current rate onto the table and exposes any gap against the incumbent's invoice. Single source removes that mechanism entirely. Nothing external tests whether the invoiced rate still matches the signed rate card, so a rate that quietly moved past its contracted terms can persist for a long time before anyone rereads the underlying agreement line by line.

This is a mechanism, not a rate of occurrence, and it applies regardless of vendor category.

3. Which contract terms need checking on a single source account?

The terms that would normally get pressure-tested by a rebid: the rate card itself, any volume tier tied to spend thresholds, escalation clauses tied to an index, and not-to-exceed caps on labor or service lines. Each sits in the contract as written on day one and drifts silently if invoices are never reconciled back against it line by line on a fixed schedule.

  • Rate card accuracy: The line-item price on the invoice should match the signed rate card, not a price the vendor updated unilaterally.
  • Volume tier placement: Spend crossing a volume tier threshold should trigger the lower rate immediately, not on the vendor's schedule.
  • Escalation basis: An index-linked increase should apply the correct index value on the correct date, not a rounded or stale figure.
  • Not-to-exceed caps: Labor or service charges should stop at the contracted cap rather than continuing to accrue past it.

4. How do you audit a single source vendor without a comparison bid?

Match every invoice against the signed contract directly, on a fixed cadence, rather than waiting for a market signal that will never arrive. This means pulling the rate card, the volume tier schedule, and any escalation or cap language, then checking recent invoices line by line against those terms. It substitutes a scheduled internal review for the review a competing bid would otherwise force.

A single source category benefits from the same audit a multi-vendor category gets by accident.

For the wider pattern this sits inside, start with the margin drift guide.

5. Frequently Asked Questions (People Also Ask)

Is single source the same as sole source?

No. Sole source means only one vendor can supply the item, usually due to patent, technology, or market structure. Single source means the buyer chose one vendor by decision, even though alternatives exist. The distinction matters because a single source relationship can be rebid; a sole source one cannot.

Is single source a bad practice?

Not inherently. It can reduce qualification cost, simplify integration, or reflect a safety or certification requirement. The risk is not the arrangement, it is the absence of a routine external check on pricing that a competing bid would otherwise provide.

How often should a single source contract be reviewed against invoices?

On a fixed schedule the buyer sets themselves, since no external event will prompt it. The exact cadence depends on contract term length and category volatility, and should be set deliberately rather than left to happen when someone notices a problem.

Does single source status affect rebate or volume tier tracking?

It can, since all volume accrues to one vendor and there is no split-vendor spend to reconcile. That concentration makes a volume tier misapplication easier to miss because the buyer has nothing to cross-check the vendor's own tier calculation against.

Can a single source vendor still overbill without violating the contract?

Yes. A rate that no longer matches the signed rate card, or a not-to-exceed cap that gets billed past, is a contract violation, not a legitimate charge. The absence of a competing bidder makes that violation harder to notice, not less real.

What is the first document to pull when auditing a single source account?

The signed contract itself, specifically the current rate card, any volume tier schedule, and escalation or cap language, checked against a sample of recent invoices line by line.

1. What is single source?

Single source is a sourcing decision to buy an entire category or part number from one vendor, with no alternate supplier qualified or invited to bid. It differs from sole source, where only one vendor exists in the market. A single source vendor is chosen, not forced, usually for switching cost, integration, or certification reasons, and the choice is a business decision, not a defect. The term describes the buyer's decision, not the vendor's market position.

2. Why does single source increase margin drift risk?

A competing bid is a recurring, informal audit: it forces a current rate onto the table and exposes any gap against the incumbent's invoice. Single source removes that mechanism entirely. Nothing external tests whether the invoiced rate still matches the signed rate card, so a rate that quietly moved past its contracted terms can persist for a long time before anyone rereads the underlying agreement line by line. This is a mechanism, not a rate of occurrence, and it applies regardless of vendor category.

3. Which contract terms need checking on a single source account?

The terms that would normally get pressure-tested by a rebid: the rate card itself, any volume tier tied to spend thresholds, escalation clauses tied to an index, and not-to-exceed caps on labor or service lines. Each sits in the contract as written on day one and drifts silently if invoices are never reconciled back against it line by line on a fixed schedule. - Rate card accuracy: The line-item price on the invoice should match the signed [rate card](/glossary/rate-card), not a price the vendor updated unilaterally. - Volume tier placement: Spend crossing a [volume tier threshold](/glossary/volume-tier) should trigger the lower rate immediately, not on the vendor's schedule. - Escalation basis: An index-linked increase should apply the correct index value on the correct date, not a rounded or stale figure. - Not-to-exceed caps: Labor or service charges should stop at the contracted cap rather than continuing to accrue past it.

4. How do you audit a single source vendor without a comparison bid?

Match every invoice against the signed contract directly, on a fixed cadence, rather than waiting for a market signal that will never arrive. This means pulling the rate card, the volume tier schedule, and any escalation or cap language, then checking recent invoices line by line against those terms. It substitutes a scheduled internal review for the review a competing bid would otherwise force. A single source category benefits from the same audit a multi-vendor category gets by accident. For the wider pattern this sits inside, start with the [margin drift](/insights/margin-drift-spend-leakage-guide) guide.

Questions & Answers

Is single source the same as sole source?

No. Sole source means only one vendor can supply the item, usually due to patent, technology, or market structure. Single source means the buyer chose one vendor by decision, even though alternatives exist. The distinction matters because a single source relationship can be rebid; a sole source one cannot.

Is single source a bad practice?

Not inherently. It can reduce qualification cost, simplify integration, or reflect a safety or certification requirement. The risk is not the arrangement, it is the absence of a routine external check on pricing that a competing bid would otherwise provide.

How often should a single source contract be reviewed against invoices?

On a fixed schedule the buyer sets themselves, since no external event will prompt it. The exact cadence depends on contract term length and category volatility, and should be set deliberately rather than left to happen when someone notices a problem.

Does single source status affect rebate or volume tier tracking?

It can, since all volume accrues to one vendor and there is no split-vendor spend to reconcile. That concentration makes a volume tier misapplication easier to miss because the buyer has nothing to cross-check the vendor's own tier calculation against.

Can a single source vendor still overbill without violating the contract?

Yes. A rate that no longer matches the signed rate card, or a not-to-exceed cap that gets billed past, is a contract violation, not a legitimate charge. The absence of a competing bidder makes that violation harder to notice, not less real.

Margin Drift Resources