Four Buckets, One Purpose
Cost of quality is not the budget of the quality department. It is the total cost a business carries because defects are possible, plus the cost it carries because defects actually happened. Every quality-related expense falls into one of four buckets: prevention, appraisal, internal failure and external failure. The classification matters more than the total, because the total alone cannot tell anyone what to do next.
The question the model answers is narrow and useful: if one unit of spending moved from failure into prevention, would total cost fall, and by how much? A factory that reports only a single quality cost figure usually finds that nobody acts on it, because no manager can see which lever to pull.
Prevention Cost
Prevention is spending that occurs before a defect can be created. It buys the ability to make the product right the first time. In zipper manufacturing it typically includes:
- Process design and control plan development for molding, stamping, plating and assembly.
- Supplier development: audits, capability reviews, joint problem solving, agreed inspection standards.
- Training and certification of operators, setters and inspectors.
- Preventive maintenance on molds, dies, plating lines and assembly machines.
Prevention is the hardest bucket to collect because it hides inside maintenance, engineering and production accounts. Data source: maintenance work orders, engineering time sheets, training records, tooling and fixture purchase orders, supplier audit reports.
Appraisal Cost
Appraisal is spending to find out whether the product conforms. It does not change the product; it only reveals its condition. It includes incoming inspection of tape, yarn, sliders and wire; in-process patrol checks; final and outgoing inspection; laboratory tests such as pull strength, salt spray and color fastness; gauge calibration; and the labour spent recording and reviewing results.
Appraisal has a natural ceiling. Beyond a point, more inspection returns less information per dollar and adds cycle time. When appraisal keeps rising while failure cost stays flat, the process is not capable and inspection is being used as a substitute for control. Data source: inspection labour hours, laboratory test logs, calibration schedules, external test invoices.
Internal Failure Cost
Internal failure is the cost of defects found before the customer sees them: scrap, rework, sorting, re-inspection, downgrading, and the disruption caused by all of it. The visible part is material loss. The larger part is usually labour and machine time consumed without producing saleable output.
Two adjustments are commonly missed. First, scrap should be valued at the cost of the work already invested, not only at raw material cost, because a molded tooth that is scrapped after plating has consumed four process steps. Second, sorting cost should be recorded even when the sorter is an existing employee, since the hours still had an alternative use. Data source: scrap reports, rework tickets, sort records, production output versus input reconciliation.
External Failure Cost
External failure is what is found after delivery: customer complaints, returns, replacement shipments, sorting at the customer site, debit notes, freight in both directions, third-party testing, and the management time consumed by the case. It also includes costs that never appear on an invoice, such as expedited production to recover a shortage and price concessions given to keep an account.
External failure is usually the smallest bucket by count and the largest by unit cost. A single container rejection can exceed a month of prevention spending. Data source: complaint log, credit and debit notes, return authorizations, freight invoices, customer scorecards.
The Formulas
The basic identity is additive. Total cost of quality equals prevention plus appraisal plus internal failure plus external failure. From that, four ratios are worth tracking monthly:
- Cost of quality as a percentage of sales: total cost of quality divided by net sales, multiplied by one hundred.
- Failure cost ratio: internal plus external failure, divided by prevention plus appraisal. A ratio above one means the business is paying to find and fix defects rather than to avoid them.
- Cost of quality per thousand units shipped: total cost of quality divided by units shipped, multiplied by one thousand. This normalizes for volume changes and makes month-to-month comparison meaningful.
- Prevention share: prevention divided by total cost of quality. A share that does not rise over a year suggests the improvement programme is not actually shifting spend.
All four need the same denominator period. Mixing a calendar month of production with a financial month of credits produces a number that moves for accounting reasons rather than quality reasons.
A Monthly Collection Routine
Cost of quality fails when it is rebuilt from scratch each quarter by one person. A short, fixed routine works better:
- Fix the period and the reporting entity before collecting anything.
- Pull scrap and rework quantities from production records by process step, not by department.
- Apply standard rates for labour and machine time, and state the rates used on the report.
- Attach complaint and debit note costs to the month the case closed, and disclose the convention.
- Separate one-off items such as a large customer sort from recurring items, and show both.
- Compare the four buckets against the previous three months and against the same month last year.
- Publish one page, with the assumptions visible, to production, engineering and finance together.
Reading the Shape of the Mix
A healthy pattern is high prevention, moderate appraisal and low failure. A common unhealthy pattern is low prevention, high appraisal and moderate failure: the factory is inspecting hard and still shipping problems. Another is low prevention, low appraisal and high external failure, which is the most expensive position because the customer is performing the inspection.
YCC publishes this breakdown by process step rather than by department, because a defect created in molding and found in assembly belongs to molding. Attributing cost to the step that created it is the only version of the report that changes behaviour. The total is a scoreboard; the attribution is the tool.
Errors That Distort the Number
- Counting only cash outflows and ignoring internal labour and machine time.
- Valuing scrap at material cost only, which understates loss after multi-step processing.
- Reporting the total without splitting the four buckets, which removes any decision value.
- Changing the labour rate or the period definition between months, which creates false trends.
- Excluding the cost of sorting performed by production staff as part of normal duties.