I agree with Ann and I will add to not confuse 'hard' with the term 'sharp' - for a sharp dog could (stress could) be viewed as aggressive, since they can be very reactive.
A wise lady on this forum once told me that having a dog that was both hard and sharp was like carrying around a loaded mouse trap in your pocket
....she gave me this analysis when helping me understand a rescued adult male rottweiler who had joined our home.......
Good luck on your paper - this forum has LOTS of information/discussions about 'hard dogs' - 'sharp dogs' - aggressive dogs, etc.......and lots of members discussing personal experiences with their own dogs.