The analysis and combination of data from different gamma-ray instruments usually involves the use of a multitude of software packages and data formats. Driven by the requirements for the upcoming Cherenkov Telescope Array (CTA) and a desire for easily reproducible results, an effort has been started to define a common data format for high-level data, namely event lists and instrument response functions (IRFs). In this collaborative project between the FERMI, MAGIC, VERITAS, FACT, and H.E.S.S experiments, a common data format was formalized and implemented. Data from observations of the Crab Nebula was made open to the public and analyzed with the open-source gammapy software. By combining data from FERMI, and the four currently operating imaging atmospheric Cherenkov telescopes, we produced a joint maximum likelihood fit of the Crab Nebula spectrum. Aspects of the statistical errors and the evaluation of systematic uncertainties are commented upon. All datasets and results presented in this work can be obtained using open-access on-line assets that allow for long-term reproducibility for everyone.