Which of the following measures a filter's ability to reject signals in adjacent channels?
Why Shape factor is the ratio of a filter's bandwidth measured far down the skirts to its bandwidth near the top, commonly the -60 dB bandwidth divided by the -6 dB bandwidth. A ratio close to 1:1 means nearly vertical skirts, so a strong signal just outside the passband is attenuated hard while the desired signal passes. That steepness is exactly what determines how well the filter rejects an adjacent channel.
Watch out Passband ripple is tempting, but it describes amplitude variation on the desired signals inside the passband, not attenuation of signals outside it; noise factor is a receiver noise measure, not a selectivity measure.
Shape factor = -60 dB BW / -6 dB BW. Closer to 1 means steeper skirts and better adjacent-channel rejection.