accuracy with which the best fit line correlates to the v co 2 and caco 2 data may be used correlation coefficients range from 0 to 1 0 where a correlation coefficient of 0 indicates that no linear correlation exists between the x ordinate and the y ordinate data and a correlation coefficient of 1 0 indicates that the x ordinate and y ordinate data are perfectly linearly correlated i e all of the v co 2 aco 2 data points are located on the same straight line the v co 2 u2014 caco 2 data points measured before and during rebreathing however are rarely located on the same straight line one reason for this is that during rebreathing maneuvers the v co 2 signal typically leads the petco 2 signal and thus the caco 2 by about one breath in addition v co 2 is calculated on the basis of signal components that have higher frequencies than do the petco 2 signal as a result when the v co 2 and caco 2 measurements calculated over a period of time are plotted against one another on a two dimensional x y line graph the result typically appears as an arc or a loop as shown in fig3 c and 5 rather than as a straight line depending on the amount of data calculated and the duration of rebreathing moreover v co 2 and caco 2 measurements may be calculated on the basis of respiratory flow and carbon dioxide pressure data obtained during spurious breaths such data do not relate to the pulmonary capillary.