What type of threat to internal validity does this present and what could the researcher do to address this issu? What threats to external validity might exist?

A researcher measures effects of accident reduction interventions in a safety program initiated in the organization among maintainers who were mandated to participate. One problem that arises is that many maintainers are deployed temporarily or transferred during the course of the study. Make sure to address the following:

1) What type of threat to internal validity does this present and what could the researcher do to address this issue?
2) What threats to external validity might exist?
Must demonstrate an understanding of the task and be able to address the requirement using creativity and application of research design knowledge.
Must demonstrate an understanding of tests for reliability and validity as used in their example.
Must include both merits and limitations that apply.