In which of the following research designs is the problem of sample attrition likely to exist? Choose the correct answer from the options given below.

Sample attrition affects longitudinal, panel, and cohort studies, which follow the same people over time.

A representative study is cross sectional and gathers data at a single point, avoiding dropout.

Longitudinal studies lose participants across repeated waves of data collection.

Panel studies track the same fixed sample, so departures accumulate.

Cohort studies follow a group sharing a characteristic and face gradual loss.

Attrition threatens internal validity when those who drop out differ from those who stay.