According to recent statistics, the mean average for all bowlers in the U.S. is between 150 and 155. While it can be done with only 5 or 6 spares, a bowler will generally throw at least 1 to 2 strikes in a 155 game. The more times you're able to get on the lanes for practice sessions, however, the higher your average will be and the more times you will be on the alley bowling strikes. With some coaching or instructional sessions, those practice sessions will be greatly improved.
You should be able to get your average between 175 and 190 within a relatively short period of time which means you are bowling strikes at least 3 to 4 times per game and converting your spares better than 60 percent of the time. More often than not, though, a person maintaining this higher average will be getting between 3 to 5 strikes per game and many times, they will be having two or three strikes (or more) in a row.
Look at it this way, with a nine pin count and spare in each of the ten frames (aka "all-spare game") the final bowling score is 190. In order for a person to get the score into the 200's, there must be at least one double (two strikes in a row) with the rest of the frames having high pin count on the first ball and making the spare. "High pin count" means 8 or 9. A 200-average bowler therefore, will generally be rolling a good amount of strikes and making more than 90 percent of their spares.
You can easily see that the higher your average goes, the more strikes are needed to maintain the average. Wouldn't you like to be a bowler who is always on the alley bowling strikes for a minimum of two, three, or four times in a row each and every game you bowl? That's what all higher average bowlers strive for.
Keep in mind that the fundamental purpose of a practice session is to improve your game so you shouldn't be worried about your score at that time. You want to concentrate on the basics of how to be consistent with your approach, your delivery, your release, your timing and rhythm, hitting your mark, and also focusing on throwing strikes and spares.
I would suggest that you categorize each of your sessions into one of the following three groups: 1) on the alley bowling strikes; 2) on the alley bowling spares; and 3) on the alley working problem areas - backswing, rhythm, sparing a certain pin, and so forth. Many higher average bowlers may throw an entire practice game concentrating on one (or more) of the aforementioned aspects.
Personally, I have practiced games where my first frame was aiming for the 5 board, the 2nd frame was at the 10 board, and so forth, until the 5th frame was aiming at the 25th board; and then, the 6th frame started me back until the 10th frame was at the 5 board again. Some other practice sessions are: alternating and throwing each of my different balls; using only my spare ball; using only one of my reactive balls for the entire game; and changing hand positions to see how the ball reacts for each position.
To be on the alley bowling strikes consistently, you will want to devote as much time as you can on improving your game. The practice session should have you working on one or two special things that you feel needs to be improved for your game to get better. You should never worry about your score when you practice.
A Hui Hou!

