why it is so important to randomly assign participants to experimental conditions

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Imagine that you are an online tutor for an undergraduate Research Methods course. One day, you get this email:
I am having some trouble understanding why it is so important to randomly assign participants to experimental conditions. It seems to me that if you have a large enough sample, the results would probably be valid even if you didn’t bother to randomize the participants. Why is it important to go through the trouble of randomizing participants? Also, if you are going to randomly assign participants, how should you do it?

Respond to this student’s email, being sure to explain the concepts at the appropriate level for an undergraduate. In your response, be sure that you explain why it is important to randomize participants. Also, explain some of the ways that a researcher can randomly assign participants. Last, be sure to explain how a researcher can make valid conclusions even in situations where randomization is not possible