In an experiment to understand the impact of fish grazing on invertebrate populations in streams, researchers established nine observation channels in three streams (three channels per stream). Each of the three channels within a stream received one of three treatments: No fish were added, Galaxias fish were added, or Trout fish were added. (The channels were constructed with mesh to prevent fish from entering or leaving.) Twelve days after establishing the channels, the number of Deleatidium mayfly nymphs present in a specified region in the center of the channel were counted. The number of nymphs for each treatment in each creek follows.16

(a) Identify the blocking, treatment (i.e., the explanatory variable of interest), and response variables in this study.
(b) In the context of this problem, explain to someone who has never taken a statistics course how blocking may help better identify treatment differences should they exist.

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Creek A B C Treatment No fish11 8 7 Galaxias944 Trout 6