In this study, the local variation of the effective channel reduction parameter ( ${\Delta }{L}={L}_{\text {m}}-{L}_{\text {eff}}$ ) of a MOSFET is extracted by means of the traditional shift-and-ratio (SAR) method. ${\Delta }{L}$ is then correlated with the threshold voltage difference ( ${\Delta }{V}_{\text {TH}}$ ) between the device under test (DUT) and the reference device. It is demonstrated that there exists an optimal ${V}_{\text {G}}$ range for extracting reliable values of ${\Delta } {L}$ through the SAR method. Statistical data analysis shows that for ${R}\approx {(}{L}_{\text {long}}/{L}_{\text {short}}{)}\approx {{25}}$ , better results are achieved since the value of $\sigma {(} {\Delta } {L}{)}$ varies typically as the reciprocal 1/ $\sqrt {W}$ . The test structure used in this work is a Kelvin-based 2-D addressable MOSFET matrix implemented in 180-nm bulk CMOS technology. The sample space is of 2304 devices divided into nine subgroups of 256 same size closely placed nMOSFETs.