I think you missed the essence of my comment which was "there is no stack ranking and being evaluated as underperforming is then not a result of stack ranking".

I'm not sure what you mean by `occasional underperformance`? I'm talking about a yearly performance evaluation, in my opinion an overall underperforming for a year is not "occasional", it shows something systemic. The truth is that sometimes we make bad hiring decision (in my group it has been 2-3%). We try to work with them for a few months, set goals, but ultimately you can't always recover everyone.

A single occurrence is never systemic.

You haven't defined what you mean by "single occurrence"? To me being low one month, or during a two months project can be a single occurrence, but that wouldn't lead to a "bad perf review" (at least not in my team). Being clearly underperforming for a complete year without changes after setting up intermediate goals for improvements is not a "single occurrence". When a Sr. Staff which is less productive than other junior engineers, and requires the same level of guidance, something isn't right.

A single bad review.