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How to measure language competence
Develop specific scales for language competence.
Because the idea of developing language skills in a certain language is so subjective, it's usually necessary for the instructor or other person who is measuring language skill to set up an arbitrary, but very important, quantitative scale.
Attach numbers to your language competence scale.
Many individuals will start with a simple scale from 1 to 10. This is a starting point for identifying and using the kinds of criteria you need to measure language competence in a person or group of people.