US non-manufacturing growth has slowed down in September

US watchers have just seen a disappointing new datapoint.

The ISM non-manufacturing purchasing managers' index (PMI) is much lower than expected - at 54.4 in September. The weakest since May. Last month it was 58.6, and analysts had been anticipating a fall - but not one this big. They were forecasting a drop to 57.4.

The sub employment sub index is also lower, at 52.7 from 57. All those points are still above 50 - implying expansion. But the rate of growth has slowed markedly.

Paul Ashworth of Capital Economics says that the drop-back "could reflect the sharp upward trend in long-term interest rates over the past few months, but that doesn't really help to explain the surprisingly big jump in August in the first place, since borrowing costs started to climb in June."

Ashworth thinks that the "jump in August always looked a little suspicious, but we didn't expect such a big reversal, particularly not after the upbeat September reading in the manufacturing survey, released on Tuesday."