Difficulty - If it is too hard, they will not succeed. The more challenging the environment or task, the slower and more complex the training becomes. Increasing the difficulty incrementally will produce learning at a faster rate.

Distance - Distance is how far away you are from your dog when they do a behavior. The farther away, the less reliable the behavior will become. When you are training distance, start small close to your dog and increase distance incrementally. 

Distractions - Anything can be a distraction. They can cause them to lose focus, become agitated, struggle to follow commands, or they may exhibit unwanted behaviors, as their attention is drawn away from you or the task at hand by sights, sounds, smells, or other people or dogs. 

Duplication - Repetition strengthens your dog’s understanding of a command or behavior. Dogs do not generalize well, so it takes many repetitions in various locations for your dog to master a skill. 

Duration - Duration is the length of time a dog maintains one or more behaviors. Shorter, more frequent training sessions are more effective for dogs. Studies indicate that shorter sessions lead to better learning and retention of commands. The optimal time depends on the dog's age, bred, and the complexity of the skill you are teaching.