@article{ref1,
title="What does operator do in complex systems",
journal="Human factors",
year="1967",
author="Christensen, J. M. and Mills, R. G.",
volume="9",
number="4",
pages="329-340",
abstract="<p>[Abstract unavailable]</p><p>Language: en</p>",
language="en",
issn="0018-7208",
doi="",
url="http://dx.doi.org/"
}