Answer to Question 1
Depending on your goals, one type of validity may be preferable if you have to sacrifice. If you are testing a theory, internal validity is more critical because you are assessing whether your theoretical ideas lead to good predictions about behavior and what causes it. If you are trying to find the effect of a single variable on behavior, internal validity will be critical.
If you are interested in applying results of research to a real world setting, external validity might be more important. You might lose some confidence in your statements of causation, but you might have more confidence in your ability to make predictions accurately.
Answer to Question 2
Attrition by children is more likely to be associated with family dynamics (e.g., the family moves, the parent(s) grow tired of participation), with children being less able to control family events. On the other hand, attrition in adults might be due to more active choices not to participate; for older participants, death may be a significant source of attrition.
external combustion engine
An adult tooth, longitudinal view showing internal structures of the crown and root.
Schematic of an automated external defibrillator (AED) attached to a patient.
(A) Internal anatomy of the spinal cord. (B) Ascending spinal tracts.
Drawing of spirochete's mobility system (with internal flagellae).
External auditory meatus