In many studies where the time period is long, participants may drop out of the study. This is which of the following threats to internal validity?

Loss of participants over a long study is the mortality or attrition threat to internal validity.

Mortality biases results when those who drop out differ systematically from those who remain.

Testing threat instead arises when taking a pre-test changes performance on a later test.

Instrumentation threat arises from changes in the measuring tool or observer over time.

Maturation threat arises from natural changes within participants such as ageing or fatigue.

Differential attrition across groups is especially damaging in long panel and experimental studies.