  We present a summary of recent tests and measurements of hadronic interaction
properties with air showers. This report has a special focus on muon density
measurements. Several experiments reported deviations between simulated and
recorded muon densities in extensive air showers, while others reported no
discrepancies. We combine data from eight leading air shower experiments to
cover shower energies from PeV to tens of EeV. Data are combined using the
z-scale, a unified reference scale based on simulated air showers.
Energy-scales of experiments are cross-calibrated. Above 10 PeV, we find a muon
deficit in simulated air showers for each of the six considered hadronic
interaction models. The deficit is increasing with shower energy. For the
models EPOS-LHC and QGSJet-II.04, the slope is found significant at 8 sigma.
