Instrumentation. Do changes in the measurement devices themselves or changes in the observers or in the way the measurement devices are used have an impact on the outcome? Example: Phyllis, a public...

Instrumentation. Do changes in the measurement devices themselves or changes in the observers or in the way the measurement devices are used have an impact on the outcome? Example: Phyllis, a public health nurse, was involved in evaluating a home health program in which she interviewed elderly clients every 4 months for a 3-year period. About halfway through, Phyllis made plans to be married and was preoccupied with this coming event. She thought she had memorized the questionnaire, but in fact she forgot to ask blocks of questions, and coders of the data complained to her that they couldn’t always make out which response she had circled on the sheets.