The KATRIN experiment aims at measuring the effective electron antineutrino mass with a 0.2 eV (90% C. L.) sensitivity by analysing the β -decay spectrum of molecular tritium near its endpoint. In May and June 2018, the KATRIN collaboration took its first commissioning data using a gaseous tritium source. These data have been used to test the stability of the entire apparatus – from the windowless gaseous source to the focal plane detector – to the 0.1% level. In addition, these first commissioning runs have been successfully utilised to test the KATRIN model and ensure the reproducibility of the fitting results, which are presented in this document.