Which is more important, skimming or scanning?

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In learning a language, we start from testbooks. Chinese students tend to be caring solely on the limited texts given to them, and maybe the piles of tests for semester, TOEFL, etc., having no initiative to explore a larger scope of books, journals, etc. to enhance their language skills. what should be the percentage of efforts or time spent respectively. I would very much like to share your opinion, for those who have english as the second language, as well as the native-speakers.