These two surveys provide the earliest national estimates each month for the previous month's economic performance. They are derived from sample surveys of buyers at major corporations. You would think they would give identical results, but they don't. That's called random variability. All economic "data" have random fluctuations. But if (if!) the fluctuations are truly random and not correlated, then averaging the two should in principle produce an outcome closer to reality. Maybe.
Anyway, the chart below shows the PMI and the ISM and the unweighted average of the two. Remember that for both times series, 50% represents a turning point. Above 50%, the economy is rising; the further above the faster. Below, the economy is contracting, and so on. My take on it: the economy continues to expand. How much longer that will continue with the Federal government shutdown is another matter. The Feb to May downturn was a direct result of the forced fiscal tightening, the so-called "sequester". A prolonged shutdown could push the US back into recession.
Disclaimer. After nearly 40 years managing money for some of the largest life offices and investment managers in the world, I think I have something to offer. These days I'm retired, and I can't by law give you advice. While I do make mistakes, I try hard to do my analysis thoroughly, and to make sure my data are correct (old habits die hard!) Also, don't ask me why I called it "Volewica". It's too late, now.
BTW, clicking on most charts will produce the original-sized, i.e., bigger version.