"We must treat individuals as individuals, not as group statistics," says Nathalie Norris' lawyer, and our heads nod in reflexive agreement. Nobody wants to be reduced to a statistic, certainly not a negative one. And yet it's hard to figure what, in practical terms, to make of the lawyer's exhortation.

Norris is the Arizona civil servant who sued when she found that her state-purchased retirement annuity paid her $33.95 less per month than her male counterparts would receive. The reason: women live longer and, as a result, giving men and women the same monthly payment for the balance of their lives would result in a higher pay-out to women.

There can be no doubt that the insurance company was treating Norris, who has taken her case to the Supreme Court, as a statistic. The tough question is one of fairness. Is it fair for an insurance company to discriminate against women? Of course not. Is it fair for an insurance company to take an actuarially sound look at its risk and set its payments accordingly? Of course it is. Insurance companies do it all the time.

Norris, it is safe to guess, never complained when her auto insurance company charged her lower premiums than it charged her male counterparts, on the ground that, statistically, women have fewer and less-serious accidents than men. Grown-ups pay smaller auto-insurance premiums than teen-agers, and teen-age girls pay smaller premiums than teen-age boys for the same reason. Is that unfair?

Listen to Norris' lawyer, Amy Jo Gitler, and it certainly sounds unfair. Why not treat insured motorists (or pensioners) "as individuals, not as group statistics"? If a particular teen-age boy is a safe driver, always observing the posted speed limits, refusing to drive after drinking, keeping his automobile in perfect working order, why should he pay a higher insurance rate than an equally safe adult or teen-age girl?

As a matter of fact, insurance companies do make adjustments for particular driver characteristics that correlate with the likelihood of accident: youngsters who make good grades, or who take driver-education courses, pay less for their insurance because, statistically, they are less likely to be involved in accidents. But surely some academically slow teen-agers who learned to drive on the Safeway parking lot will have better reflexes and better driving habits than their statistically safer counterparts. The trouble is, no insurance company can afford the cost of examining every applicant in sufficient detail to set a rationally deduced individual premium. Willy-nilly, we are all reduced to statistics.

And sometimes it hurts.

If Nathalie Norris had taken either of two additional options available to retired Arizona civil servants, there would have been no basis for her complaint. She could have taken her pension in a lump sum that would have been the same for all similarly situated retirees. Or she could have taken the money in a fixed number of monthly payments, again without gender distinction. But she opted to use her retirement money for a state-purchased annuity that would produce a monthly check for as long as she lived. Since, statistically, she was projected to live longer than her male counterparts, the company made her monthly check smaller.

Is that discrimination? If she had been a black man, with a statistically shorter life expectancy, would it have been reverse discrimination to make her monthly payments larger? Would she buy a solution that equalized pay-outs for men and women by reducing the size of the payments to men? What is the reasonable way out of the dilemma she has raised?

Nathalie Norris says she is a victim of sex bias. It may be more accurate to say she is a victim of common sense.