The seventh week of the seventh season of "Playing Football" spotlights seven selections. At stake is an imaginary $2,600, which could buy a lot of non-fiction. The price are:

St. Louis, $100, getting 3 1/2 points at Washington; San Diego, $250, giving 14 1/2 at home against the New York Giants; Cleveland, $250, giving 9 at home against Green Bay; Houston, $250, giving 9 at home against Tampa Bay; Pitssburgh, $250, giving 10 1/2 at home Monday night to Oakland; Baltimore, $750, giving 2 at home against New England, and Philadelphia, $750, giving 3 at home against Dallas.

This represents the heaviest investment of the season and only the third week in which a sizable sum has been risked. The reason for the cautions approach should be obvious. There are many questions to be answered about every team every year. It takes time to get the answers. Form is much more likely to take a beating during the formative period than later on, when the betting line is more refined.

All of which is extremely important to my approach, which concentrates on the stronger teams in the league. I generally stay away from Green Bay, the New York Jets and Giants, etc., no matter how attractive they are made to look by their "getting" one or two touchdowns or more. These may be teams to bet on, by picking their opponent to embarrass them, but they are never to be trusted in a positive sense, as the means of winning a bet.

I have, over the years, concentrated on the top 10 or 12 teams. They are much easier to chart. They are more dependable. Their performance is more predictable. This, in turn, makes predicting against the point spread more profitable.

The weaker teams, by their very nature, are going to play unevenly. Even their coaches don't know what to expect.

There is no bigger sucker in the football betting market than the person lured into taking a team simply because it is getting 10 or more points.

Go with the strength, whenever possible, but be careful the spots you pick. I still prefer handicapping a game involving two strong teams, where the spread is small, over a game that has a strong team against a weak team and a big number.

Dallas at Philadelphia and New England at Baltimore this week provide two excellent examples of strength against strength. All four teams should make the playoffs.

New England's offensive line is second to none. But Baltimore's is terribly underrated. Points will be plentiful, so it comes down to the quarterbacks Steve Grogan versus Bert Jones -- as to which one is more likely to complete a critical pass. I'll take Jones.

The key matchup at Philadelphia concerns Dallas' defensive front, the best in the NFL, working against the eagles' underrated offensive line. If Randy White, Harvey Martin and Too Tall Jones can get to Ron Jaworski frequently, he'll wilt. I'm betting they won't -- and that Cowboy quarterback Danny White might not make fans remember Roger Staubach in his first high-pressure outling.

The home field often provides an extra reason for making a selection. This season, however, home teams are 34-50 against the spread. But in games of obvious importance, such as the two mentioned above, playing at home is an even greater consideration. Even the officials can get caught up in the roar of the crowd.

Concerning the other selections:

San Diego should be ashamed of its performance the last two weeks and be ready to take out its frustration on the sad-sack Giants. Cleveland's offense is just now getting into gear, with Green Bay coming to town. Houston is ready to bust out, what with Earl Campbell recovered from his groin miseries. Pittsburgh will find out in a hurry if Jim Plunkett has overcome his fear of crowds. St. Louis' splendid running back, Ottis Anderson, should add to the Redskins' woes.

In other games, Las Vegas lists Atlanta 3 over New Orleans, Buffalo 3 over Miami, Detroit 2 1/2 over Chicago, Denver 5 over Kansas City, Los Angeles 9 over San Francisco, Cincinnati 4 over Minnesota and the Jets 2 1/2 over impossible-to-fathon Seattle.