An unusually large revision to the mid-month PMI flash perhaps hints at an upward surprise for the ISM manufacturing index at the top of the hour. The PMI ends September at 49.8 which is up 9 tenths from last week's flash and up 1.9 points from August.

Still this is the fourth straight sub-50 reading which has hovered near or below this breakeven mark since October last year. Econoday's consensus for the ISM, which has been a degree lower in the 46 to 48 range over this same period, is a steady reading at 47.8.

New orders contracted at a slower rate for the PMI sample and output rose marginally. The sample added workers in September reflecting the best optimism for the 12-month outlook since April last year. Inflation assessments, though strengthening, remain muted.

The final manufacturing PMI for September is expected to come in at 48.9, unchanged from the mid-month flash to indicate modest contraction from August.

Based on monthly questionnaire surveys of selected companies, the Purchasing Managers' Manufacturing Index (PMI) offers an advance indication on month-to-month activity in the private sector economy by tracking changes in variables such as production, new orders, stock levels, employment and prices across manufacturing industries. The final index for the current month is released roughly a week after the flash.

Investors need to keep their fingers on the pulse of the economy because it dictates how various types of investments will perform. By tracking economic data such as the ISM manufacturing index in the U.S. and the Markit PMIs in the U.S. and elsewhere, investors will know what the economic backdrop is for the various markets. The stock market likes to see healthy economic growth because that translates to higher corporate profits. The bond market prefers less rapid growth and is extremely sensitive to whether the economy is growing too quickly and causing potential inflationary pressures.

The Markit PMI manufacturing data give a detailed look at the manufacturing sector, how busy it is and where things are headed. Since the manufacturing sector is a major source of cyclical variability in the economy, this report has a big influence on the markets. And its sub-indexes provide a picture of orders, output, employment and prices.

Markit originally began collecting monthly Purchasing Managers' Index (PMI) data in the U.S. in April 2004, initially from a panel of manufacturers in the U.S. electronics goods producing sector. In May 2007, Markit's U.S. PMI research was extended out to cover producers of metal goods. In October 2009, Markit's U.S. Manufacturing PMI survey panel was extended further to cover all areas of U.S. manufacturing activity. Back data for Markit's U.S. Manufacturing PMI between May 2007 and September 2009 are an aggregation of data collected from producers of electronic goods and metal goods producers, while data from October 2009 are based on data collected from a panel representing the entire U.S. manufacturing economy. Markit's total U.S. Manufacturing PMI survey panel comprises over 600 companies.