DESCRIBE the four different threats to internal validity [intact groups, extraneous variables, subject loss, and expectancies]. How do these threats apply to the following study:

You have randomly selected 300 students to participate in your research study. In this project, 100 subjects are asked to take a placebo, another 100 will take a drug that has been shown to enhance memory, and a newer experimental drug that you believe will be a miracle breakthrough in the field of memory research. Students are blind to which drug they are given, and they continue to take this drug every day for the ten week term. Due to financial restrictions, you are unable to hire someone to track the students, and you are not blind to which group the subjects are in. However, you are determined to remain objective and unbiased at all times. At the end of the ten week period, all subjects are given a test that covers information from the entire ten week course. Subject scores on this exam are compared and contrasted, and you find that subjects who were taking your new drug outperformed both the placebo and the standard memory drug. What conclusions can you make from this finding?

Of the four different threats to internal validity [intact groups, extraneous variable, subject loss, expectancies], which one do you think is the biggest threat to internal validity? Why? Is external validity an issue here too?