For this reason, it is absolutely critical that KRs be specific enough to be measurable (this is where KPIs come in), and that they contain a numerical element that makes them easier to form metrics for. In fact, one technique to ensure KR accuracy is creating KRs according to the S.M.A.R.T. method. The S.M.A.R.T. method dictates that goals should be: Specific; Measurable; Achievable; Relevant; and Timebound.
Source: Trillion-Dollar Teamwork: Goal-Setting With OKRs – DZone Agile