Availability of duck food in summer. Ducks inhabiting the Great Salt Lake marshes feed on a variety of animals, including water boatmen, brine shrimp, beetles, and snails. The changes in the availability of these animal species for ducks during the summer was investigated (Wetlands, March 1995). The goal was to compare the mean amount (measured as biomass) of a particular duck food species across four different summer time periods:
(1) July 9-23, (2) July 24-Aug. 8, (3) Aug. 9-23, and (4) Aug. 24-31. Ten stations in the marshes were randomly selected, and the biomass density in a water specimen collected from each was measured. Biomass measurements (milligrams per square meter) were collected during each of the four summer time periods at each station, with stations treated as a blocking factor. Thus, the data were analyzed as a randomized block design.
(a) Fill in the missing degrees of freedom in the randomized block ANOVA table shown in the next column.
(b) The F-value (and corresponding p-value) shown in the ANOVA table, part a, were computed from an analysis of biomass of water boatmen nymphs (a common duck food). Interpret these results.
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SOURCE
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df
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F
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p-VALUE
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Time period
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-
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11.25
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0.0001
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|
Station
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-
|
-
|
-
|
|
Error
|
-
|
|
|
|
Total
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39
|
|
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(c) A multiple comparisons of time period means was conducted using an experimentwise error rate of .05. The results are summarized below. Identify the time period(s) with the largest and smallest mean biomass.
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Mean biomass (mg/m2):
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19
|
54.5
|
90
|
148
|
|
Emotional state:
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8/24 - 8/31
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8/9 - 8/23
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7/24 - 8/8
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7/9 - 7/23
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