Answer to Question 1
Answer: External validity refers to the extent to which the results of an experiment can be generalized to other circumstances, such as other populations or places. Internal validity is the degree to which an experiment allows confident statements about cause and effect. Experimenters often are forced to make a trade-off between external and internal validity. The reason is that externally valid studiesthose that are most generalizable to the world outside the laboratorytend to be less controlled (and therefore have less internal validity) as a result. Conversely, experiments that have high internal validity often are so well controlled that they produce a very artificial environment, and therefore are more limited in the degree to which they generalize to the outside world.
Answer to Question 2
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An adult tooth, longitudinal view showing internal structures of the crown and root.
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Internal structure of the nose—lateral view
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