Figure 4. Normal probability plot of transect data.
Figure 5. Normal probability plot of fixed monitor
(Straight line plots the normal distribution; square
station data. (Straight line plots the normal
symbols are the observed data.)
distribution; square symbols are the observed data.)
analysis. The data essentially fit the normal distribution line, but show a trend to be above the
line for higher cumulative probabilities and below the line for lower cumulative probabilities. We
conclude that the data are approximately normally distributed.
Figure 5 suggests that the fixed monitor data are also normally distributed. Note that the data in Figure
5 are somewhat more randomly distributed on each side of the normal line, with fewer "runs" or
continual observations on one side or the other of the normal line. This graphically based determination is
subjective. To lessen subjectivity, tests as discussed above can be used (Kolmogorov-Smirnov, Lillefore's,
etc.), but the analyst is often forced to use whatever data are available.
Because the data were collected for another study and not specifically for monitor verification, the
transect locations did not coincide exactly with the monitor location. For our comparison, all transects
that were within 3.5 km of the fixed monitor station were selected. The number of samples varied with
transect mile and date. The May samples had five or seven evenly spaced measurements at a constant
depth of 4.6 m. July samples had multiple depths and five to seven sample locations. The calculations
of the differences, the square of the differences, and the totals of the two sites are depicted in Table 2.
Table 2
Differences (D), Squares of Differences (D2), and Totals for Data Specified
in Table 1 (Sample Size, n = 8)
D2
Date
Transect Mile
D
18 May 95
119.9
1.7
2.9
25 May 95
121.2
2.6
6.8
25 May 95
121.6
1.7
2.9
25 May 95
122.1
0.9
0.8
25 May 95
119.9
3.6
13.0
27 Jul 95
121.2
2.3
5.3
27 Jul 95
121.6
4.1
16.8
27 Jul 95
122.1
3.4
11.6
Total
20.3
60.1
7
Water Quality Technical Note AM-03 (January 1998)
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