The threat to internal validity is the Mortality. Internal validity is the measure in which the causality of the stimuli studied is analyzed, that is, if the results of the study are due to the interaction of the variables analyzed or are due to the interaction with other variables. Mortality is an error that occurs if the conclusions of the study are made only on the basis of the participants who were from beginning to end in the investigation, without taking into account those who left the study.
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