Well, the PMI are actually surveys of companies polled every month. Not just any companies. We must build many replicas of the economy. And we do this by obtaining official data on the true structure of the part of the economy that we want to survey.
Here's an example: The is US manufacturing. We can see the contribution to total output in the US manufacturing sector, by categories such as food and drink, textile and clothing. And within each category, we can see the correct mix of small, medium, and large firms that we need to accurately reflect the true structure of manufacturing. Here is the equivalent data for the US service sector: hotels, restaurants, and within them, the mix of small, medium, and large service sector includes transport and storage, telecommunications, financial intermediation, the banking sector, business services, and consumer services. It's a huge disparate part of the economy. Importantly, these matrices that we use to build these survey panels we update every year to make sure they're accurately reflective of the economy at any given time. And of course, they will change from country to country. No two countries’ manufacturing sectors look the same. We take that into account when we're building our panels.