One condition is the frequency. Frequency, in swimming, is the number of times you swim or take swim class during a period of time; let's say a week. If Jimmy has kids swim lessons every day after school, he might learn to swim faster than Susie, who only has swim lessons every Saturday. Unless Susie goes to the pool to practice every day after school with her mom. In this case, Jimmy and Susie might learn to swim at the same rate. Frequency can also mean the number of times you perform a certain stroke during a period of time. If Jimmy works on front crawl, or free style, more because he doesn't like being on his back, and Susie works on her back crawl more than her front crawl because she loves being on her back, they might have both learned a stroke at the same rate, but they learned different strokes.
In effect, they have learned something at the same rate but they both learned something the other doesn't do well, and yet they are still both at the same level of knowing how to swim. Confusing? Let's just say that the more times the kid performs a certain action, the more likely they are to pick up on that action and learn how to correctly execute it, with little help.
Frequency depends on many other things however, such as environment. Now, you probably think that the environment for a swim class is the swimming pool. You would be right, but that's not all the environment is. It also includes the classmates, the parents watching the class, the toys or equipment used, other swimmers in the pool, distractions like kids playing on the deck or other classes in the pool, and the instructor teaching the class. Each one of these can affect how fast or what a class actually learns. Let us pretend that Jimmy's class is very peaceful. None of the kids cry for their mommy, the kids in the class pay attention and don't goof off, and they only touch the equipment when the instructor tells them to. Basically a swim instructor's heaven. In this class, the skills can be taught and reviewed in an orderly manner, and be reviewed often. The class will most likely progress fairly quick and leave time for them to play that wonderful game of sharks and minnows.
On the other hand we have Susie's class, in which Billy wants his mom who has left, all the kids play around with the equipment, no one pays any attention, and by the end of class the instructor wants to hold herself under the water while a shark bites on her heel and a sting ray whips her senseless. This is a normal swim class on a bad day. There is so much distraction that no one pays attention and therefore does not know what to do, resulting in a class that progresses slowly and unfortunately does not get to play any games (oh darn). So you see, a class that has a good environment will most likely progress more rapidly than a class with a more cluttered environment.
We would like to blame it on the rest of the class or the "class clown," but sometimes the environment and frequency are not the only things that keep the kid from progressing faster. What other factors might have an effect on how fast kids learns how to swim?
American Red Cross Swimming and Water Safety Book
American Red Cross Fundamentals of Instructor Training Participants Manual