Geoscience Reference
In-Depth Information
(Intercept)
Year
90
89
88
87
86
85
84
83
82
81
0.1
0.2
0.3
-0.02
0.00
0.02
0.04
FIGURE 11.4
The 95% confidence intervals on the intercepts and slopes for each of the stations in the mer-
cury concentration data set.
right of the vertical dashed line had statistically significant positive trends
over the study period (i.e., stations 82, 86, 87, 88, and 90). The most substan-
tial increase in mercury concentration is observed with station 90, where the
estimated slope is β=
ˆ
1,90 , implying that the mean mercury concentra-
tion is estimated to be increasing at a rate of 0.0317 ppm per year. Given this,
over a 10-year period the expected increase in concentration at station 90
would be 0.317 ppm.
Although the unit analysis provides direct insight into the trends for each
of the individual sampling units, it is useful to make a general statement
about the entire study area from which the study sites were selected. To make
these types of statements with the unit analysis, summaries of the regression
parameter estimates across each of the sampling units can be calculated. As
an example, the average trend is estimated to be
0.0317
10
ˆ
ˆ
β=
β
/10 0.0131
=
(11.2)
1
1,
i
i
=
1
where ˆ 1 denotes the overall average trend across the sample of stations. Thus,
on average, for the study area monitored, the mean mercury concentration
Search WWH ::




Custom Search