A boundary inside a number.
In a fraction, the denominator is the number below the line. For a completion rate, it identifies the population against which completions are counted.
“80% completed” leaves a question unanswered: 80% of whom?
80 / 100 = 80%among people who started
80 / 140 ≈ 57.1%among everyone invited
Ask before comparing.
Do both rates use the same population, definition of completion and time window? A change in who is counted can change the percentage even when every outcome stays fixed.
An empty frame has no rate.
When the denominator is zero, the percentage is undefined. Reporting 0% would imply a populated group with no completions, which is a different situation.
Source & scope
Marecek and Anthony-Smith, Prealgebra, Chapter 4: Key Terms (OpenStax, 2015) defines numerator and denominator. The completion example is an original constructed illustration, not data from that book or a real programme.
The demonstration does not explain selection, missing records or reasons for non-participation. Those require evidence beyond a fraction.