According to the May issue of Consumer Reports, consumers are volunteering a large amount of information about themselves to retailers and other groups that have an interest in consumer demographics.

Many stores ask shoppers for their telephone numbers and addresses so they can be added to the firm's mailing list, says the magazine. Questionnaires appear on everything from warranty cards to cereal boxes. People who bought Cap'n Crunch cereal, for instance, were offered high-value coupons and a wristwatch for filling out a questionnaire. Quaker Oats wanted to know consumers' opinions on firearm ownership, school prayer and mandatory drug testing.Families that take part in supermarket programs that track purchases on a computer connected to the store's bar-code scanners get cents-off coupons, and marketers find out everything from what deodorant Dad prefers and what diapers baby uses to how many soft-drink six-packs the family consumes each week.

Such information is used to compile a demographic profile of consumers who use particular products.

What can you do if you want to protect your privacy?

The magazine makes these suggestions:

- If you're applying for credit, read the disclosure statement before you sign on the dotted line. That way you'll know how much privacy you're about to give up.

- Keep your Social Security number to yourself. Never write it on a check or credit-card report.

- If a merchant insists on a phone number or address on a credit slip, you can refuse. There's no law that requires this information. And the major credit-card companies discourage or even prohibit merchants from asking.

- Check credit reports for inaccurate information. It's important to do particularly before applying for a big loan.

- Write to any of the major credit bureaus to opt out of pre-approved credit-card offerings.

- If you want to have your name removed from mail-order company mailing lists, write the Direct Marketing Association, P.O. Box 3861, New York, NY 10163-3861, and ask to have your name removed. Shady operators are not members of this organization, however.