Let's take the case where you're trying to create a Key Result where the start and target value are the same number, for example maintaining a 0% Net Attrition Rate.
First and foremost, if you're looking to maintain a metric, it sounds more like a KPI than a KR. More information on the difference between OKRs and KPIs here.
However, if you want to calculate the progress in this case, a measurement as a way of checking that what you're doing isn't breaking anything, you have a couple of options:
Measure it as a binary 0/1, yes or no in terms of completion (0 if you didn't maintain it and 1 if you did over the quarter).
Or, if it's something that you measure more frequently, then you can have a scale based on the time you measure it. For example, if weekly would be 12 times, it could be 0/12. And the concept remains the same: you get a point if you maintain it and don't get a point if you don't.
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