Problems with Randomization
Randomized trials are expensive
Sometimes randomized trials can be unethical. Example: We can not randomize the population of interest into people where they are forced to smoke for testing if there is a causal relationship from smoking to lung cancer.
Randomized trials consume a lot of time for testing and collecting results. Sometimes the results are needed urgently and waiting for the outcome is not an option. Following people for a longer duration of time can be hard.
It's hard to build the perfect environment for testing and controlling the environment. Example: testing medication on people and making sure the consumption of medication and all other things are controlled is hard.
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