To achieve accurate heart rate variability (HRV) analysis using wearable electrocardiogram (ECG) recording devices under daily life environment, ideally all erroneous R-R intervals (RRIs) comprising misdetected R wave (i.e., false positive, FP) and overlooked R wave (i.e., false negative, FN) should be corrected by RRI editing. In the development stage of RRI editing, we need to assess errors at the RRI unit level between reference RRIs and test RRIs (hereafter called an RRI evaluation pair) regardless of the presence or absence of FPs and FNs. However, conventional RRI error assessment methods such as visualization and one-axis measures do not achieve a balance between quantification and applicability. Hence, there is currently no method to quantitatively assess errors at the RRI unit level when the test RRI sequence contains FPs. In this study, we propose a two-tier RRI error assessment method. As the first tier, we propose a rule for creating the RRI evaluation pairs regardless of the presence or absence of FPs and FNs, which uses the time gap between a reference RRI and a test RRI. As the second tier, we propose using the two-dimensional vector of the RRI evaluation pairs in the RRI tachogram (vector in the x-y plane) in consideration of the geometric characteristics of erroneous RRIs comprising FPs and FNs therein. Our experimental results showed that the proposed RRI error assessment method combined with the pairing rule was able to quantitatively assess errors at the RRI unit level by the magnitude of the vector, regardless of the presence or absence of FPs and FNs. Also, the quadrant of the vector direction indicated whether the error was FP or FN: the second quadrant corresponded to FP, and the third and fourth quadrants corresponded to FN. Since our proposed method uses only time-related information, it can be applied regardless of the method used for R wave detection and RRI editing. Hence, the method may be used to compare several RRI editing methods that assume different situations in terms of FP or FN.