In the past, BASE jumpers used parachutes when making their high-altitude leaps. In the past few years, wingsuits have become popular; they generate lift, allowing the jumper to glide rather than free-fall, but cause landings to take place at much higher speeds. The concern that wingsuits are more dangerous than parachutes is clearly sustained by the fact that the number of BASE jumping injuries was much higher last year than it was ten years ago.
Which of the following, if true, most seriously compromises the strength of the evidence cited?