Each of the following can be threats to external validity EXCEPT:

Biology Forums - Study Force is a free online homework help service catered towards college and high school students. Get homework help and answers to your toughest questions in biology, chemistry, physics, mathematics, engineering, accounting, business, humanities, and more. Master your assignments quickly with thousands of step-by-step solutions to countless textbook questions asked and answered by our members. If we don't have your question, don't worry. You can ask any homework question and get expert homework help in as little as two hours.

Our extensive online study community is made up of college and high school students, teachers, professors, parents and subject enthusiasts who contribute to our vast collection of study resources: textbook solutions, study guides, practice tests, practice problems, lecture notes, equation sheets and more. With our help, your homework will never be the same!

Join for free today  

How is Biology Forums - Study Force different than tutoring?