Quality (PPM)Quality Metric for Parts per Million

Basis of Calculation

The most common formula is: defect quantity divided by the total ordered delivery quantity, multiplied by 1,000,000. If 1.2 million screws are delivered in a year and 60 parts are rejected, this results in a value of 50 PPM. What is decisive is that numerator and denominator are collected consistently – for example, only via incoming goods inspection or including defects from production.

• Numerator: rejected or sorted-out part quantity
• Denominator: total quantity delivered or inspected in the period under review
• Factor of 1,000,000 for normalisation

Distinction from Other Quality Metrics

PPM describes the defect frequency but says nothing about the severity, cause or cost of a defect. For this, supplementary metrics such as complaint rate, scrap costs or First Pass Yield are necessary. Material traceability also plays a role, so that defects can be clearly assigned to a batch or supplier.

Supplier Evaluation and Sourcing

In supplier evaluation systems, PPM often carries a weighting of 20 to 40 per cent in the overall score. In supplier acquisition and in qualification criteria for new suppliers, the historical PPM value serves as hard evidence of process capability. Suppliers who can demonstrate low PPM values thereby also improve their chances in tenders and volume commitment agreements.

Contractual Anchoring

Many procurement contracts contain PPM target values with agreed escalation levels, such as a warning, a special sorting inspection (sorting action at the supplier's expense) or contractual penalties. A clear definition of the measurement method in the contract is important in order to avoid disputes. Legal statements on defect rights should always be examined on a case-by-case basis.

Data Sources and Collection

Typical data sources are incoming goods inspections, measurement records, 8D reports and complaint management systems. Ideally, these are fed directly from ERP or quality management systems, for example via item master records and posting documents linked through Material Requirements Planning (MRP). A solid material availability check (ATP) also prevents quality problems from being concealed by substitute procurement.

Defect Classification and Delimitation

Not every raised issue is a genuine quality defect: short deliveries, incorrect deliveries and transport damage should be recorded separately. A uniform defect classification – such as critical, major, minor – increases comparability across suppliers and plants.

Root Cause Analysis with the Supplier

If the value exceeds agreed limits, a structured reconciliation process with the supplier is customary, often in the 8D report format. In addition, a supplier feasibility review can reveal whether the intended processes can sustainably deliver the required quality.

Key Metrics in Combination

In addition to delivery PPM, on-time delivery, complaint resolution time and quality costs are particularly relevant. Together, these metrics show whether a problem is episodic or systemic.

• Delivery PPM (based on incoming goods)
• Field PPM (defects at the end customer, often only visible months later)
• Quality costs (sorting, rework, freight costs)
• Response time to complaints

Interpretation and Guideline Values

In practice, values below 500 PPM are considered good for many series deep-drawn and machined parts, while automotive-critical components often require values in the single-digit range. However, guideline values must always be related to the part class, manufacturing process and risk potential – blanket thresholds lead to incorrect conclusions.

Typical Measurement Errors and Biases

Common sources of error: small batch sizes, which statistically produce hardly meaningful PPM values; unclear delimitation between purchased-part defects and production defects; double counting of the same defect in incoming goods and production. Even in the risk analysis prior to supplier selection, the measurement methodology should therefore be clearly defined.

Control Mechanisms in the Procurement Process

Effective controls include regular data reconciliation between procurement, quality management and production, defined escalation paths and periodic audits at critical suppliers. A supplier change should only be considered once improvement measures have demonstrably failed – because every change carries its own quality risks during the ramp-up phase.

Documentation and Traceability

Complete traceability of materials down to batch number is a prerequisite for assigning defects and making corrective actions effective. Without clean traceability, costly demarcation problems up to and including recalls loom in the event of a defect.

Automated Data Collection

Modern ERP and quality solutions record inspection results directly at incoming goods and calculate PPM values almost in real time. E-procurement in procurement also creates seamless electronic documents that make it easier to assign complaints to purchase orders and make the small-order process more visible from a quality perspective.

Preventive Rather than Reactive Control

The trend is moving from downstream complaint handling towards preventive quality assurance: process capability studies, should-costing and clean-sheet calculations are increasingly used to identify unrealistic quality requirements in advance. Those who recognise early that a price is only achievable with a reduced inspection depth avoid later PPM outliers.