When I turn down a social invitation during March, I try to be vague about my reason for saying no. I have round that candor is seldom the best policy. It just leads to arguments.

When a poker crony asks whether I'll be at the weekly game I say, "I don't think so. The frammis on my graffle plate isn't working and I'll have to take it apart and clean it Wednesday night." This may be slightly misleading, but it does convey the message that I will have to hurry home to my mortgage on Wednesday and some other student of statistical probabilities will therefore have to be recruited to fill in for me. That's all my friend really needs to know.

The truth is that I will be spending Wednesday night, and many others, trying to locate, classify and evaluate the financial data that will serve as the basis for filling my income tax return.

However, it is a mistake to tell anybody that this is how I will be spending my time because the rejoinder always runs something like this: "You make out your own tax return? Man, that's the dumbest thing I ever heard of. I turn mine over to an expert. He does all the work, and he charges such a modest fee compared to the amount he saves me that I actually come out money ahead on the deal. You must have rocks in your head if you make out your own returns."

Yes, I have rocks in my head. Now that you know, shouldn't you stop trying to persuade me to act rationally?

I don't insists that you make out your own tax return; you shouldn't insist that I use the services of your tax expert. Each of us ought to be willing to let the other make his own mistakes.

For me, there is an important principle involved in making out my own tax return. I have a strong feeling that the tax structure in a democracy ought to be simple enough for an ordinary citizen to understand.

When the tax structure becomes so complicated that even the legislators who created it cannot understand it, the consequence should be widespread errors and even some noncompliance, not widespread expenditure for the services of professional tax experts. If most Americans filed inaccurate returns, the Congress would at long last be forced to simplify its tax laws.

Then, too, there are practical reason for not employing expert help. The finest expert in the land can't do anything for me until I gather up all the pertinent records that, as Gold's Law notes, are never filed where they are supposed to be. Getting the raw data together is the most time-consuming part of filing a Form 1040. The arithmetic is relatively simple.

Another good reason for not turning to an expert for help is that the kind of "expert" most individuals can afford is one who works only occasionally at this seasonal occupation, and one whose skill is therefore suspect. When one needs quotation marks to describe the "experts" expertise, there is good basis for wondering: "What do I need this guy for? Can't I read the instructions as well as he can?"

Large corporations and wealthy individuals need the services of genuine experts who are better versed in the intricacies of the tax laws than any layman can hope to be. Large corporations and wealthy individuals can afford to hire genuine experts.

However, the average, he can calculate his tax as well as an average part-time "expert" can. Better, perhaps.

If you think I overstate the case, check with the Montgomery County Office of Consumer affairs. OCA's director, Barbara Gregg, says she recently sent a male and female investigator to nine tax preparers, where they posed as a typical couple seeking help with their income tax forms. The investigators provided each of the nine tax preparers with the same financial data but got back eight different answers. An independent tax consultant given the same basic facts said the "couple" should get a tax refund of $247. The commercial tax preparers turned in "bottom line" figures that ranged between a refund of $222 and an additional tax owed of $408 - a swing of $630. Since state taxes are based on the federal return, the preparers' errors would have cost the "couple" as much as $703 - plus the preparer's fee.

Why should I buy this kind of expertise? I can read instructions, and I take a greater interest in my return than anybody else might. So enjoy your poker game, fellows, or whatever else strikes your fancy. I'm going to be busy for another couple of weeks.