AS and THAN are like oil and water. They really don't mix.

Making comparisons correctly can be tricky in writing. Many writers use comparative words that do not work well together. Here is a good example from a sentence in this morning's newspaper. It is from an article for Bloomberg News http://www.bloombergnews.com by Nicole Ostrow.



Those who used the drug for six or more years were twice as likely to develop a psychosis such as schizophrenia or to have delusional disorders than those who never used marijuana.



When using AS at the beginning of a comparison, use AS rather than THAN in the second part of the comparison. This writer was probably trying to avoid using AS twice because she had the phrase "such as schizophrenia," but the comparison is still awkward. I would also use the word MARIJUANA first and then use THE DRUG in the second part of the sentence. It could be written correctly this way:



Those who used marijuana for six or more years were twice as likely to develop a psychosis like schizophrenia or to have delusional disorders as those who never used the drug.


THAN can be used in comparisons, but it won't work in this sentence because of the phrase TWICE AS LIKELY. Here are examples of using THAN correctly in comparison:



Those who turned the essay in early were more likely to get a good grade than those who turned it in on time.



Those who used salt ate more french fries than those who did not.



Those who chose Door Three were less likely to win the big prize than those who chose Door One.



Hope that helps.