Radio signals from a distant transmitter should get stronger and then weaker as the telescope's focal point moves across that area of the sky. Specifically, the power should increase and then decrease with a bell shaped curve (a gaussian curve). Gaussian curve-fitting is an excellent test to determine if a radio wave was generated "out there" rather than a simple source of interference somewhere here on Earth, since signals originating from Earth will typically show constant power patterns rather than curves. The Gaussian test is only applied for frequency resolutions greater than or equal to 0.59 Hz.
In the above figure, "power" tells us how strong a Gaussian is relative to noise. "Fit" is a measure of how well a rising and falling signal fits an ideal Gaussian (bell curve) profile. A lower "fit" value means a better fit. (It's actually a chi-square fit, i.e., a measure of how far the data departs from an ideal Gaussian.) Since noise can sometimes randomly simulate a Gaussian, the SETI@home screensaver only returns Gaussians stronger than 3.2 times the average noise level with a fit less than 10.
For more detailed information on Gaussians, see:
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SETI@home and Astropulse are funded by grants from the National Science Foundation, NASA, and donations from SETI@home volunteers. AstroPulse is funded in part by the NSF through grant AST-0307956.