Mental chronometry (MC) studies cognitive processes measured by time. It provides an absolute, ratio scale. The limitations of instrumentation and statistical analysis caused the early studies in MC to be eclipsed by the "paper-and-pencil" psychometric tests started by Binet. However, they use an age-normed, rather than a ratio scale, which severely limits the ability of IQ tests to probe the physical basis of differences in cognition. For this reason, Arthur Jensen reinitiated mental chronometry in the 1970s. He designed an apparatus that measures reaction time to a task known as the Hick paradigm that requires the testee to respond to a display of 1 to 8 lights. Faster decision times were related to psychometric "g," with theoretically important consequences. He was able to do this, where many other studies had failed, mainly because his apparatus clearly separated movement (MT) from reaction time (RT, also called "decision time".) Interestingly, while RT is clearly related to IQ, MT is not. Principal components analysis reveals RT to be a cognitive variable and MT a motor variable. Failure to distinguish between them drastically obscures the correlation between composite RT (i.e., RT + MT) and cognitive variables. When Jensen (2006) reviewed the literature on MC he found there was a shocking lack of standardization in the administration, recording, and analysis. Consequently, the results of a study conducted in one lab, even though measured in absolute time, could not be compared directly against those from another. Termed "method variance," this is a major obstacle to the advancement of MC. For that reason, Jensen's Institute of Mental Chronometry commissioned a leading electronics firm to construct a state-of-the-art apparatus to administer, record, and analyze MC experiments. (Contains 1 figure.)