The second half of the year has begun, and any information that can shed more light on the health of the economy is welcome.

One of the best leading indicators of the economy is the purchasing managers index (PMI). At the beginning of each month, Markit, HSBC, RBC, JP Morgan and several other major data gathering institutions publish the latest local readings of the manufacturing PMI for countries around the world.

"Collectively, these numbers will give investors a critical insight into the pace of June economic growth," says JP Morgan's Adam Crisafulli.

Each reading is based on surveys of hundreds of companies. From Markit:

PMIs are based on monthly surveys of carefully selected companies. These provide an advance indication of what is really happening in the private sector economy by tracking variables such as output, new orders, stock levels, employment and prices across the manufacturing, construction, retail and service sectors.

The PMI surveys are based on fact, not opinion, and are among the first indicators of economic conditions published each month. The data are collected using identical methods in all countries so that international comparisons may be made.

These are not the most closely followed data points. However, the power of the insights is unparalleled. Jim O'Neill, Chairman of Goldman Sachs Asset Management, believes the PMI numbers are among the most reliable economic indicators in the world.

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