1.28 = number of standard errors necessary to reach an 80% 
level of confidence 
STANDARD ERROR = 1.2533 (ANE) 
AN 
By establishing a confidence interval on the basis of the sample, one 
can express with 80% confidence the range within which the true 
median lies. 
DISCUSSION 
Obviously, interpretation of the statistics generated for each World 
Aeronautical Chart depends on the size of the sample. As shown in Table I, 
a large percentage of the charts contain insufficient data to permit detailed 
conclusions. 
TABLE | 
DATA DENSITY AND DISTRIBUTION 
wore NORTHERN HEMISPHERE || SOUTHERN HEMISETIERE 
Seuss) vee |e eee ees ee 
Of the 931 charts comprising the southern hemisphere, 814 or over 87% contain 
less than 50 data points. Although the northern hemisphere has been sampled 
somewhat better, 433 or over 47% of the 920 charts still contain less than 50 
data points. Because of the limited number of observations both the charted 
isogonic lines and the statistics of this evaluation must be weighed carefully in 
