25th and 75th percentile depth changes are -2.3 and +2.5 feet; 

 thus in 50 percent of the cases the change is less than 2.4 feet (in 

 absolute value) in a horizontal distance of 304 feet. The 15th and 

 85th percentile changes occur at -4. 7 and +4. 8 feet, with 70 per- 

 cent of the data in this range. The corresponding vertical angles 

 are less than 0°27' (in absolute value) for the central 50 percent of 

 the cases and less than 0°54' for the central 70 percent of the data. 

 The example is a nearly typical case because the median of the 

 absolute values of the slope of more than 65, 000 data samplings is 

 0°25', and the 70th percentile of the absolute values of the slope is 

 0°51'. 



As a measure of depth-change variability in the entire area, 

 the values of depth changes per 304 feet, which corresponded to 

 the 25th and 75th percentile, were scaled from each of the cumu- 

 lative percentage distributions (Appendix B). The absolute values 

 were averaged as was done in the single example (fig. 17). The 

 15th and 85th percentiles were also determined and averaged. 

 This is analogous to the "significant wave" method whereby the 

 upper 30 percent is "average." However, these values represent 

 a depth change greater in absolute value than 70 percent of the 

 observations. 



As an alternate treatment, a cumulative percentage distribu- 

 tion curve could be plotted for the absolute values of depth differ- 

 ences. The new 70th percentile change would agree almost exactly 

 with the average of the absolute values of the 15th and 85th per- 

 centiles. Hence the average will be designated 70th percentile of 

 absolute value of depth change (absolute -value 70th percentile — 

 depth change). Likewise, the 50th percentile in an alternate treat- 

 ment would agree with the average of the absolute values of the 

 25th and 75th percentiles, and that average will be designated 

 absolute-value 50th percentile — depth change. 



DISTRIBUTION OF SLOPE 



The frequency distribution and cumulative frequency of all 

 slope data on isotherms, both shallow and deep, are presented 

 (fig. 18). The cumulative frequency is based on 65, 000 data points 

 and summarizes the isotherm slope in the entire area around the 

 tip of Baja California. The cumulative frequency curve shows that 



39 



