They both try to measure the quality of the hip joint, but they use somewhat different measurements and systems for rating those measurements.
OFA rates the hips with an overall qualitative statement: excellent, good, fair, dysplastic. Fair and up means no sign of diseases, but good or excellent is better because it reflects a better shape to the socket and ball joint of the hip. You mail one radiograph of the extended hips to OFA, and three vets examine the image to produce the official rating.
- OFA's advantages are that it's easy to understand, proven to reduce the incidence of dysplasia, and available in a publicly verifiable database automatically when you pass (or when you fail but also check the box that allows failing results to be published).
- OFA's disadvantages are that if you send in a poorly positioned x-ray, you can end up with a worse rating than your dog really deserves. Also, in order to be a simple, easy to understand system, it doesn't give you all the information you might want about the joint.
PennHIP uses a quantitative measurement of hip laxity (how loose the joint is) in order to predict disease. It uses three different positions and radiographs (extended, compressed, and distracted). It creates a number called the distraction index that you can use to compare hips in a more specific way than OFA's qualitative ratings. This is slightly oversimplified, but basically, the higher the distraction index, the looser (and more disease prone) the hip. You also get the hips put on a percentile scale against other PennHIPed dogs in the breed, do you know where your dog's DI falls.
- The advantage is that you get a clear, specific measurement of the hip's laxity.
- The disadvantages of PennHIP are that doesn't give you a clear cutoff line on what's good to breed or what's not the way OFA does. It also doesn't post the results in a publicly verifiable database (though you can have PennHIP results cross posted to OFA for a fee, I think).
Both are considered excellent ways to help breeders reduce hip problems in their breed by ensuring breeding stock is free of disease. OFA is more straightforward and publicly verifiable, and it's currently the gold standard (pun intended) for Golden breeders. Lots of breeders use PennHIP on top by having all the radiographs taken at the same time (one of the three PennHIP x-rays is the same one you'd use for OFA). That way you get a rating from OFA you can post publicly, and also the more specific information from PennHIP that might be helpful for an experienced, knowledgeable breeder who's trying to create impeccable hips for their canine athletes.