Determining whether a landing is hard using QAR data should be done with great care. The air/ground switch is usually recorded at a low sample rate on the QAR which will affect the results if you are looking g loads at impact. Strictly hard landings should not be based on g-loads but on sink rate exceedance. However in practice g-loads are used or pilot reports. I would recommend to have a look at this article by Boeing:
http://www.boeing.com/commercial/aer...onditional.pdf.
Personally I favor firm landings especially on wet/flooded runways to promote wheel spin-up. Soft landings often tend to be long landings too...