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…
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.