English translation: block release / sandwich course
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Explanation: COMMENT ONLY, to have room to explain
My own empirical observations lead me to regard it more as a question of 'ratio' than of actual frequency, though I think 'sandwich' generally tends to refer to shorter periods — for example, to take a local case at the college where I work, we talk about en alternance' for students who are doing 3 weeks at work and 1 week at college — to me, that's a 'sandwich course'
But where people are away for possibly longer blocks of time, and possibly more nearly equal ratios, I believe it might become 'block release' — for example, ½ a term spent at college and ½ at work.
I think there is a degree of blurring of the boundaries, I've never personally heard any definitive definition of one vs. the other.
Here's what NS OED says about 'sandwich' as an adj.:
"Of a training course etc.: consisting of alternating periods of theoretical instruction in a college etc. and practical experience with a firm etc. "
"thick sandwich (course): a sandwich course with an extended period of formal college instruction between two shorter periods of practical experience in industry etc."
and for 'block release":
"block release release of a person from his employment for the whole of a stated period to attend a course of study"
...which seems to play down the alternating back-and-forth aspect of it