A very basic definition of a service or assistance dog in many jurisdictions is that the dog performs tasks that mitigate your disability. This is just one of the characteristics that make a suitable service dog.
In Home Tasks
If you are considering in-home work only (your dog only works at home for you), almost any healthy dog of suitable size for the task can be taught some simple tasks. Your dog can even be ‘stubborn’, fearful or even what you consider to be dumb if you apply learning theory! Once your dog learns how fun it is to learn and that there is no risk in trying new things, you can both have fun learning together and can progress to more complex tasks. All it takes is a little time everyday and executive functioning to carry it out.
If you want your dog to also accompany you in public to mitigate your disabilities, (certified or not) your dog needs to have a sound temperament, be well-socialized, be in good physical shape, and be an appropriate size for the tasks you are requesting. The dog MUST have been well-socialized to people, other dogs, animals and all environments you plan to go as a working team. Basically, having solid behavior in public is the foundation of any dog used as a service dog for public access. The Canine Good Neighbor test run by the CKC or the CLASS program run by the APDT is a good way to determine if your dog might have the basic training needed to start working in public and SDTI also does in-person evaluations.
It also depends on what tasks you want your dog to do.
A hearing dog, for example, needs to be alert to sounds and active enough that he is willing to jump up from a sound sleep to let you know someone is knocking on your door or that your morning alarm is sounding. On the other hand, a mobility dog does better if they have a calm enough temperament that they can lay under your chair until you ask him/her to help you. A corgi would not be suitable to help you brace yourself as you stand because it would put too much strain on his back, and it would be hard for a large mastiff to retrieve small dropped objects without mouthing them.
It’s really about having a good match between the dog, person, situation, lifestyle, and tasks required.
To see if your dog might have potential for public access or if you are considering selecting a dog for service work, choose for temperament, health, size, exercise and grooming requirements, not by breed (mixed breeds can do very well). Even within a particular breed, individual temperament (avoid pups with fearful or aggressive parents), health and exercise needs vary.
To get a good idea of if a specific dog is suitable for service dog work, SDTI does in-person assessments in the Nanaimo, B.C. area and also can help you assess a dog via webcam if you take your hand-held device with you and have bandwidth or wifi on site where the dog or puppies are. Dogs over 18 months are the better choice if you are choosing an adult dog as their temperament is consistent after that point, unless the dog experiences trauma or illness.
For puppies, it pays to look closely at the parents, grandparents and what the breeder does with the pups in the first 8 weeks of life. Their experience in that time can help to start their life off as a service dog. Look for breeders that raise their pups with the Puppy Culture or Avi Dog programs.
Research shows that temperament tests are a better indication of what the breeder has or hasn’t done with the pups, much more than predicting what the pup’s future temperament will be since life experience and the environment a pup lives in has a large effect on each pup and their genes.
More than 50% of assistance dogs trained by organizations are removed from the program before graduating. Fearful dogs are among the first to be declined. Health issues, aggression, over-friendliness and too high drive are other reasons dogs fail as service dogs. Working in public requires a team (dog and handler) to be held to higher standards as public safety is at stake. The failure rate among owner-trained dogs is much, much higher with some teams failing with two and three dogs before either giving up or having the right dog, the right teaching skills (theory and practical), patience and good teamwork.