What is the threats to the internal validity of longitudinal design?

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What is the threats to the internal validity of longitudinal design?



Answer: Conversely, only one group of subjects is used in a longitudinal study, but order cannot be counter- balanced, and the world "gets older" at the same time as the subjects, so any difference in behavior across ages might be due to the changes in the world, instead, which is known as a time-frame or zeitgeist effect.

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