OP has a scientific background. - LOL - I have an HND (technician) in Civil Engineering and I am now the MD of an IT consultancy (obvs). I picked up the habit of using s-1 etc when studying Physics 'A' level (UK) many, many moons ago. Not too sure why I persist with it these days but I dimly remember liking the fact that you can use basic arithmetic on superscripts. To be fair I should put s^-1 but s-1 is reasonably obvious.