Pluses and minuses in having your own investment computer

A personal computer will never replace the stockbroker (or so stockbrokers hope), but it could give individual investors one of the broker's most important tools: more and faster information, instant access to stock and option quotes, and the ability to respond to those quotes with buy or sell orders.

Within a relatively short time, seven new computerized investing systems have been introduced. So far, these seven don't have many customers to share. The best estimate of the number of people who can pad downstairs in their pajamas at midnight and look into their portfolio, as well as the refrigerator, is less than 5,000.

This small beginning is partly because many people don't have a personal computer, and many of those who do would prefer to deal with a live broker on the telephone. For the rest, questions about investment patterns, costs, and record-keeping have yet to be answered.

Before plunking down a few hundred to a few thousand dollars for the equipment, figure out what kind of investor you are. If you make only a few trades a year, or if you're apt to react - or overreact - too quickly to changes in the markets, investing via home computer is probably not for you.

Most of the people signing up for these systems, company executives say, are active investors who make at least four trades a month. They want to follow their portfolios closely - as well as a selection of other stocks - without having to call their broker and ask for a dozen or more quotes every few days.

''If you're a long-term investor who is perfectly happy getting your information from the newspapers,'' you probably don't need one of these systems, says Norm Nicholson, editor of Computerized Investing, a bimonthly publication of the American Association of Individual Investors.

In fact, Mr. Nicholson adds, being able to trade with a keyboard could quickly pile up commission costs, perhaps wiping out any gains made from moving in and out of the market.

Most people who use home computers for investing, Nicholson says, don't have time to keep close track of their portfolios during the day. They tend to be professionals who are too busy with daytime jobs to be calling their broker during business hours. Although some retirees are included, Nicholson's statistical profile describes a profesional man who is 51 years old and earning more than $65,000 a year.

While computer-based services appeal to upper-income individuals, you don't need a fancy $2,000 or $3,000 personal computer to use one of them, says Eric Korben, group marketing director at Fidelity Investments, one of the firms offering a system. ''I do it with a $200 terminal and a $65 black-and-white TV, '' he says. As long as you also have a modem to connect the terminal to telephone lines (Mr. Korben's is built in), you have all the equipment you need.

If you want to spend more on equipment, you can get a computer that will accept investment analysis programs, the programs themselves (known as software) , a couple of disk drives (one for the analysis, the other for market data), and a printer for written confirmation records. By this time, of course, you probably have invested $2,000 or $3,000 in computer equipment.

In addition to this, of course, is the cost of the service itself. Most of the services have a $49.95 or $50 sign-up charge, plus a $15- to $30-a-month access charge. That fee allows up to one hour of cumulative use; there is an additional charge for any extra time. Some services also charge more for access during weekday working hours.

Another way to cut - or raise - costs is to decide between ''real time'' quotations or a 15- to 20-minute delay. With real time, you can watch your stock change price almost as soon as a trade is completed and recorded. The delayed quotes come indirectly from other brokers instead of the exchange.

At Fidelity, you can get real-time stock and options quotes for an additional cost an extra $20 a month. Leaving out the more expensive options quotes is the main reason Spear's real-time service is cheaper, says Charles Spear, president of the firm, which is offering the service in conjunction with a computerized news and information service called the Source. Spear Securities began service Nov. 1.

Even if you have a computer to help with investing, you'll never be able to buy and sell stocks directly. That has to be done through authorized brokers and traders. Also, all trades are supposed to be be reviewed by brokers before they are transmitted to the floor of an exchange. The Securities and Exchange Commission requires the broker to double-check with the customer if he has any question about an order - such as a large order for a highly speculative issue from someone who has always stuck to blue chips.

Besides choosing a computer and an investing service, you may also want news and research information. As noted above, Spear Securities includes the Source in its real-time stock quotes service. Max Ule, a New York-based service, offers CompuServe Information Services, which features news from Standard & Poor's and the Associated Press. Separate services, like Dow Jones News/Retrieval, are also available. Some of these services let you analyze possible future effects of various investment choices on your portfolio.

This leads to another advantage of investing by computer: trial runs. Before actually placing an order, or while learning about a new investment like options , you can work up a mock portfolio of investments and keep a constant watch on what might happen if you were play ing with real money.

Freddie Mac's security

Could you comment on the investment quality of Freddie Mac certificates as compared with any good-quality bond? My wife and I own about $50,000 in these, and we have noticed that certain banks and savings-and-loan associations must think fairly well of them, judging by the investment each has made in them.- G. D.

Of the three major US government-sponsored housing-finance corporations - Freddie Mac (Federal Home Loan Mortgage Corporation), Fannie Mae (Federal National Mortgage Corporation), and Ginnie Mae (Government National Mortgage Association) - only Ginnie Maes are backed by the ''full faith and credit'' of the US government, like Treasury bonds and bills, or US Savings Bonds. But the others, including Freddie Mac certificates, are considered virtually as safe, because they involve sponsorship or guarantees by these government agencies, and Congress would almost certainly not allow one of them to default on existing debt. Because of the slightly higher risk, however, yields on the others are usually a bit higher than on Ginnie Maes.

Worthless stock write-off

I have a stock that became worthless this year, and I have a letter from my broker stating it has no value. Does this capital loss have to be taken this year, or can it be postponed until 1985?- J. W.

It sounds as if you'll have to declare the loss this year. Any loss like this must be taken in the year the stock is declared worthless, an Internal Revenue Service official says. The only way a postponement might be allowed would be if the company declared bankruptcy or went out of business, but legal proceedings or some other final resolution of the corporation's affairs delayed the actual declaration of stock worthlessness until the next year.

If you would like a question considered for publication in this column, please send it to Moneywise, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115. No personal replies can be given. References to investments are not an endorsement or recommendation by this newspaper.