After we teach a dog to do something (anything: sit, stay, come when called, jump, retrieve, etc), the next part of the training process is to 'proof' that behavior.
Proofing a behavior is the process of creating a generalized response from the dog; That's trainer jargon for making sure the dog will perform a command in a variety of circumstances, with a variety of distraction. A dog who has generalized a behavior will reliably perform that behavior no matter what.
During the proofing process, we want to ensure that the dog not only understands their job, but is also motivated to do the work. Teaching each behavior in a systematic way can do both, or neither. It's the approach that matters. Many trainers believe that systematically training a new behavior involves increasing the levels of distance, distraction, and/or duration each time they ask the dog to perform. By example, when teaching the 'sit,' the handler continuously asks more of the dog each repetition, each time creatively thinking of new and more challenging scenarios for the dog. In this chart, on a scale of 0 to 10, 10 being the most difficult scenario one can imagine for the dog, the common approach is example A:
Don't make the mistake of progressively adding difficulty to EVERY repetition. Dogs will begin to recognize the pattern in Example A. Over time they understand that each successful repetition of the behavior will result in a new and even more challenging scenario. In a reward-based training system, even if your dog is one of those coveted food or toy obsessed types, the result can be a dog who decides the reward isn't worth it. After all, the more he gives the more you ask of him. This is especially true of a dog who's motivation is on the medium or low end of the spectrum.
To maintain motivation to perform, a better strategy for proofing a behavior is to change the pattern of difficulty by repeating a lower level after a successful repetition of a more challenging scenario (Example B). We are still systematically increasing difficulty, but we break up the hard repetitions with something easier, so the dog doesn't become deterred by ever mounting pressure to perform.
Let's use the command 'sit' as an example. Assuming I have done the ground work and my dog will sit on a verbal cue at a level 1 of each of the 3-D's, Where do I go from here? The first step is to develop a training plan.
Work on Duration first. Mitigate the other two D's by staying close to the dog, and limiting distractions. Systematically add duration to each repetition, in a way that maintains motivation (as in the Example B chart). Next, mitigate duration and distance, and systematically work on levels of distraction; again, using concept in Example B. Lastly, mitigate distractions and duration, and strictly work on distance using the same approach. Now you are ready to create scenarios where all three D's are a factor, and you can repeat the process once more.
The chart is just a reference point. It may take more than 15 repetitions for your dog to really 'get it.' The main point is to maintain motivation, by challenging the dog in a way that also allows for maximum success.