This paper presents a physical interpretation of linear regression comparisons between measured and predicted values of the polar cap potential and the interplanetary electric field (IEF). We define a geoeffective scale size in the solar wind LG as spanning all interplanetary equipotentials that touch the magnetopause. Simple dimensional analysis considerations suggest that LG is proportional to the slope of the linear regression curve. The regression intercept represents potential from sources other than the IEF. Four independent data sets indicate that LG is only a few RE wide in the YGSM direction. This has implications for the reliability of space weather forecasting. Most IEF observations from subsolar libration orbits are taken many RE from the Sun--Earth line. Good correlations with near-Earth variations require that the IEF has a YGSM scale size that is significantly larger than LG. ¿ 1999 American Geophysical Union