different heart rates. To account for this, and thereby improve the reliability of QT measurement, the QT interval can be corrected for heart rate (QTc) using a variety of mathematical formulae, a process often performed automatically by modern ECG recorders. Bazett's formula The most commonly used QT correction formula is the Bazett's formula, named after physiologist Henry Cuthbert Bazett (1885–1950), calculating the heart rate-corrected QT interval (QTcB). Bazett's formula is based on observations from a study in 1920. Bazett's formula is often given in a form that returns QTc in dimensionally suspect units, square root of seconds. The mathematically correct form of Bazett's formula is: where QTcB is the QT interval corrected for heart rate, and RR is the interval from the onset of one QRS complex to the onset of the next QRS complex. This mathematically correct formula returns the QTc in the same units as QT, generally milliseconds. In some popular forms of this formula, it is assumed that QT is measured in milliseconds and that RR is measured in seconds, often derived from the heart rate (HR) as 60/HR. Therefore, the result will be given in seconds per square root of milliseconds. However, reporting QTc using this formula creates a "requirement regarding the units in which the original QT and RR are measured." In either form, Bazett's non-linear QT correction formula is generally not considered accurate, as it over-corrects at high heart rates and under-corrects at low heart rates. Bazett's correction formula is one of the most suitable QT correction formulae for neonates. Fridericia's formula Fridericia had proposed an alternative correction formula (QTcF) using the cube-root of RR. Sagie's formula The Framingham correction, also called as Sagie's formula based on the Framingham Heart Study, which used long-term cohort data of over 5,000 subjects, is considered a better method. Again, here QT and QTlc are in milliseconds and RR is measured in seconds. Comparison of corrections A retrospective study suggests that Fridericia's method and the Framingham method may produce results most useful for stratifying the 30-day and 1-year risks of mortality. Definitions of normal QTc vary from being equal to or less than 0.40 s (≤ 400 ms), 0.41 s (≤ 410 ms), 0.42 s (≤ 420 ms) or 0.44 s (≤ 440 ms). For risk of sudden cardiac death, "borderline QTc" in males is 431–450 ms; and, in females, 451–470 ms. An "abnormal" QTc in males is