If it is impossible for a researcher to find an unaffected subgroup to use for a control, the use of a one-group before-after design might be unacceptable because of the reactive effect of the before observation.In such cases, a separate random sample is sometimes interviewed for the before measure.

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This is true. When it is impossible to find an unaffected subgroup for a control, a separate random sample can be used for the before measure to avoid the reactive effect of the before observation in a one-group before-after design.

If It Is Impossible for a Researcher to Find an Unaffected