“…1, there were 6 testing levels, which were represented by more than one primary study (including the unit testing). There were papers focused on functional testing (7 papers, e.g., [20], [34], or [80]), integration testing (6 papers, e.g., [26] or [43]), system testing (7 papers, e.g., [25], [29], or [49]), regression testing (2 papers - [39] and [77]), and vulnerability testing (2 papers - [52] and [83]). There were also 4 other testing levels, each represented by a single primary study (4 papers, e.g., [22] or [64]).…”