Figure of merit

A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives.

Examples

• Clock rate of a CPU
• Calories per serving
• Contrast ratio of an LCD
• Frequency response of a speaker
• Fill factor of a solar cell
• Resolution of the image sensor in a digital camera
• Measure of the detection performance of a sonar system, defined as the propagation loss for which a 50% detection probability is achieved
• Noise figure of a radio receiver
• The thermoelectric figure of merit, zT, a material constant proportional to the efficiency of a thermoelectric couple made with the material
• The figure of merit of digital-to-analog converter, calculated as (power dissipation)/(2^ENOB × effective bandwidth) [J/Hz]
• Luminous efficacy of lighting
• Profit of a company
• Battery life of a laptop computer
• Heat absorption and transfer quality for a solar cooker
• Audio amplifier figures of merit such as gain or efficiency

Benchmarks are synthetic figures of merit that summarize the speed of computers in performing various typical tasks.

Modulation systems

In modulation systems for communication, figure of merit of a device means the noise figure_C).

Amplitude modulation

Figure of merit for amplitude modulation (AM) is given by

$\mathrm {\frac {(SNR)_{O,AM}}{(SNR)_{C,AM}}} ={\frac {k_{a}^{2}P}{1+k_{a}^{2}P}}$

Figure of merit for double-sideband suppressed-carrier (DSB-SC) receiver or that of a single-sideband (SSB) modulation is always unity. Therefore, noise performance of AM receiver is inferior to that of a DSB-SC receiver or an SSB receiver.

Frequency modulation

Figure of merit for frequency modulation is given by

$\mathrm {\frac {(SNR)_{O,FM}}{(SNR)_{C,FM}}} ={\frac {3k_{f}^{2}P}{W^{2}}}$

Deception

The precision and verifiability of numbers sometimes make them a more effective sales tool than vague and non-numeric descriptions such as "state of the art" or "leaves the others in the dust". When used in deceptive advertising, the deception lies more in the question of relevance rather than truth since the number quoted, as a figure of merit may not be enough to determine performance when comparing products. For example, when purchasing a laptop a consumer could choose on the basis of the capacity of its hard drive. The RPM, buffer, and seek times may not be noted, but may significantly affect performance.

Some figures such as peak music power are used in selling consumer merchandise and have the principal merit of yielding high numbers that can impress people who do not know what the numbers mean. Other figures such as specific fuel consumption are addressed to engineers and other studious buyers whom the sellers dare not mislead.

Another example is the megapixel count of a digital camera. A consumer unaware that the number of pixels on a sensor is only one factor in the quality of the image that is captured may, for example, buy a camera with more pixels squeezed onto a smaller image sensor, thus losing quality to the small pixel size.

Makers of cheap, consumer-market telescopes often tout the magnification power of their products, sidestepping the fact that aperture, optical quality, and the type and quality of the telescope's mount are more important for obtaining a quality image.

See also

• Performance metric