To conduct an appropriately randomized experiment, it's important to choose a population that allows for a controlled study where participants can be randomly assigned to different diet groups. Among the three populations you mentioned:
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Dogs in California: This population is too broad, as it includes various breeds, sizes, ages, and individual characteristics that could affect activity levels.
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Golden Retrievers in Omaha, Nebraska: This population is more specific, focusing on a single breed in a particular location. It allows for a more controlled environment, but it still has geographical limitations.
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Poodles in the United States: This population grouping is also focused on a specific breed but is national in scope. Like Golden Retrievers in Omaha, this group is more homogenous than the first option.
While both Population #2 and Population #3 could potentially be used for a randomized experiment, Population #2: golden retrievers in Omaha, Nebraska would likely be the most appropriate choice. This is because it is specific enough to limit variability due to differences in breed (Golden Retrievers only) and provides a manageable geographic area to conduct the experiment. This specificity helps reduce confounding variables that could arise from differences in breed or location.