EXAMPLE 1 Dependent or independent sampling?

Indicate whether each of the following experiments uses an independent or dependent sampling method:

  1. A study was designed to compare the differences in price between name-brand merchandise and store-brand merchandise. Name-brand and store-brand items of the same size were purchased from each of the following six categories: paper towels, shampoo, cereal, ice cream, peanut butter, and milk.
  2. A study was designed to compare traditional acupuncture with usual clinical care for a certain type of lower-back pain.2 The 241 subjects suffering from persistent nonspecific lower-back pain were randomly assigned to receive either traditional acupuncture or the usual clinical care. The results were measured at 12 and 24 months.

Solution

  1. For a given store, each name-brand item in the first sample is associated with exactly one store-brand item of that size in the second sample. Therefore, the items in the first sample determine the items in the second sample. This is an example of dependent sampling.

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  2. The subjects were randomly assigned to receive either of the two treatments. Thus, the subjects who received acupuncture did not determine those who received clinical care, and vice versa. This is an example of independent sampling.

NOW YOU CAN DO

Exercises 5–8.