The makers of bathroom scales would have you think that improved technology has made personal weight monitoring an exact science. But the most striking technological advance usually appears only in the displays. Many scales, including almost half the 39 models tested by Consumer Reports, show your weight with electronic digits instead of a pointer or a dial. Mechanical workings often reside beneath the electronic display.

In a spring scale, the most common type, levers under the platform transmit the force of your weight to a calibrated spring, which stretches in proportion to the weight imposed. The spring makes the scale's dial or pointer move, or governs the reading on a digital display.

The only type of scale that can rightfully claim to be electronic is the strain-gauge. When you step on a strain-gauge scale, your weight applies force to a small steel beam that bends by small degrees. As it deflects, it stretches a wire, causing a change in the wire's electrical resistance that's proportional to your weight. A microchip monitors the resistance, translating it into a signal for the digital display.

A third type of design, represented by the Medixact Hydraulic 6400, $65, uses neither stretching springs nor straining metal beams. Stepping on the Medixact's platform forces fluid from one compartment to another. The fluid's pressure deflects the pointer by an amount proportional to your weight.

Your exact weight probably matters less than the changes revealed from one weighing to the next. So testers considered consistency -- the ability to give the same reading for a given weight time after time -- the most important attribute of a scale.

With some models, testers found that one sample would perform quite differently from another -- so credit was given to those scales that were consistently consistent, the ones that varied least from sample to sample. The top-rated Health O Meter 840, $50, didn't have the highest scores for accuracy or repeatability, but it did show high sample-to-sample accuracy and high overall performance.

The results of Consumer Reports' tests found that digital displays generally were easier to read than analog dials. Most digital models have red LEDs (light-emitting diodes), with lighted numbers against a dark background. The highly rated Thinner MS-7, $50, has an LCD (liquid crystal display) readout, with dark numbers against a light background. Although both types show up well in normal lighting, LEDs are clearer in dim light and LCDs are clearer in bright light. With lighted numerals 2 inches high and an inch across, the display of the Health O Meter 190, $79, was by far the easiest to read.

Some analog displays feature a dial that turns around a fixed needle. Others have a needle that spins around a fixed dial. Neither type has an inherent advantage, so far as can be seen. And neither type of analog display is as easy to read as the numerals in a large digital display.

Analog scales have the advantage of always being ready to use; step right on, and your weight awaits you as soon as the dial or pointer settles down. In contrast, most digital scales require more preparation to energize their electronics: You must first activate the scale by using a toe to tap the platform, push a button or move a small lever. Only when you see zeros appearing in the display -- usually within three seconds -- do you know that the scale is ready to weigh you.

Two high-rated models with a spring weighing mechanism are the Borg Digital 9855, $23, and its near-twin the Counselor Digital 850, $22. They both were judged Best Buys.

Overall, tests showed that a brand name or a designation such as "professional" doesn't guarantee high performance. Some "professional" scales weren't as good as their nonprofessional counterparts. And the quality of different models from the same manufacturer also was found to vary. Different Health O Meter models, to take but one example, appear at the bottom as well as at the top of the ratings.