PMI Diffusion Index Explained
All four Business Activity indicators in this vault (ISM Manufacturing PMI, ISM Services PMI, S&P Global Manufacturing PMI, and S&P Global Services PMI) are built on the same Purchasing Managers' Index (PMI) framework. Understanding it once lets you read all four.
All four Business Activity indicators in this vault (ISM Manufacturing PMI, ISM Services PMI, S&P Global Manufacturing PMI, and S&P Global Services PMI) are built on the same Purchasing Managers' Index (PMI) framework. Understanding it once lets you read all four.
What Is a PMI?#
PMI is a diffusion index: it measures the breadth of change across a sector, not the magnitude. Each month, purchasing managers at hundreds of companies are asked one simple question per topic: is this activity higher, the same, or lower than last month?
The Formula#
How to Read the Number#
| Reading | Meaning |
|---|---|
| Above 50 | The sector is expanding |
| Below 50 | The sector is contracting |
| Exactly 50 | No change |
The further from 50 in either direction, the broader the expansion or contraction across surveyed firms.
ISM vs S&P Global: Why Two Providers?#
Both ISM and S&P Global publish Manufacturing and Services PMIs for the U.S. They sound identical but serve different purposes.
| Feature | ISM | S&P Global |
|---|---|---|
| History | Since 1948 — the legacy U.S. standard | Newer — the international standard |
| Geographic scope | U.S. only | Same methodology across 40+ countries |
| Company focus | Large multinational companies | More medium-sized businesses |
| Market preference | Fed and domestic traders pay heavier attention | Used for cross-country comparisons (U.S. vs Europe vs Asia) |
| Manufacturing panel | ~400 companies across 18 NAICS industries | ~600 companies |
| Services panel | ~400+ companies across many NAICS industries | ~400 companies |
| Manufacturing components | 5, equally weighted at 20% each | 5, unequally weighted |
| Services headline | 4-component composite index | Single business activity question |
| Potential bias | Skewed by international events (large multinationals) | Slightly more reflective of domestic U.S. economy |
Practical rule of thumb:
- Watch ISM for domestic U.S. business cycle signals and Fed reaction.
- Watch S&P Global when comparing U.S. versus overseas economic momentum, or when you want an earlier mid-month Flash signal.
Why PMIs Are Leading Indicators#
ISM states that diffusion indexes have the properties of leading indicators. The intuition:
- Manufacturers must order raw materials and ramp up production before finished goods reach store shelves.
- Service firms must hire and accept new business before revenue is realised.
- Purchasing managers therefore see the next quarter's economy through their order books before official GDP or employment data can confirm it.
S&P Global similarly states that PMI data are widely used to anticipate changing trends in official data such as GDP, industrial production, employment, and inflation.
The New Orders sub-index is consistently the most forward-looking PMI component: it captures demand for future production, before the production itself happens.