Start-up runaway electrons are one of the key challenges for ITER. Unlike disruption generated RE beams, the experimental data analysis in present does built on dedicated experiments but on existing databases of many shots as in most devices start-up RE occur naturally in certain percentage of discharges. On the other hand this allows us to map the effects of various parameters only in a small, natural range. Furthermore the initial phase of the discharge is often poorly diagnosed and controlled and once suitable conditions are found, operators often do not allow variation of the optimal parameters. Thus, dedicated experimental data with parameter scans motivated by physics understanding is in fact hard to obtain. This contribution aims on initiating the discussion on the best methods to analyse start-up RE databases and missing steps in applications and comparison with numerical models. An analysis of start-up REs in COMPASS is presented with the early electron density being apparently the most important parameter. The relation between early non-thermal ECE and early HXR is examined. An initial look on the ASDEX-U start-up RE database is also presented.