The science has been studied and the results have shown that positive reward based training is the most effective in the long term and leads to stronger bonding. Ironically, a good dog trainer will get results very quickly that are longer lasting because the dog is not only enjoying the training, but he/she is doing the behaviors themselves and not being forced into doing them.
So here is my advice as a professional dog trainer. When looking for someone to hire to train your dog, for the sake of your dog's wellness, find someone who is science-based and utilizes positive reinforcement and has credentials. Ask if they are a member of the Pet Professional Guild (PPG).
Don't be afraid to ask the trainer if they use E-collars or prong collars. Ask them if they have had any formal training with many of the different positive science-based dog training schools. Check out their reviews online and see what previous clients have said about them. Are they using painful, brutal and fear based tactics? Or do they subscribe to a positive based reinforcement methodology. If any red flags pop up during any of these questions, take your time, money, and your canine friend elsewhere. Trust me, these impostors are not worth your time.
If you wanted to find the right surgeon to do heart surgery on a family member, wouldn't you want to make sure they are qualified? The same applies to dog trainers. Ask the questions you need to know before you hire them and they irreversibly damage your dogs psyche. Dogs are not born mean or aggressive. They become this way due to old school training tactics that should have died out decades ago once the scholarly articles started coming out.
Do your research. Protect yourself. And Protect your dog from these crooks who will take your money and leave you with bigger problems in the future.