Diffusion Index

A diffusion index is the arithmetic engine inside surveys like the PMI. Imagine 100 purchasing managers are asked whether new orders rose, stayed the same, or fell this month. Suppose 40 say rose, 20 say fell, and 40 say unchanged. The diffusion index adds the "rose" share plus half the "unchanged" share: 40 + 20 = 60. A result above 50 means more respondents reported improvement than deterioration; below 50 means the reverse.

The logic behind splitting the "unchanged" group in half is that stability is neutral — it contributes equally to neither expansion nor contraction. This convention is why 50 is the universal neutral line for PMI-style surveys. Traders typically watch whether a reading is above or below 50, and whether it is rising or falling, rather than focusing on the precise number.

A key thing to understand is that a diffusion index measures breadth — how widely spread a change is — not its magnitude. If most factories are growing but only slightly, the index can still read 60. Conversely, a small number of very large factories could be surging while most small ones stagnate, and the index would not fully capture that imbalance. For that reason, economists often read diffusion indexes alongside hard output data like industrial production figures for a fuller picture.