  We present an analysis of comet activity based on the Spitzer Space Telescope
component of the Survey of the Ensemble Physical Properties of Cometary Nuclei.
We show that the survey is well suited to measuring the activity of
Jupiter-family comets at 3-7 AU from the Sun. Dust was detected in 33 of 89
targets (37 +/- 6%), and we conclude that 21 comets (24 +/- 5%) have
morphologies that suggest ongoing or recent cometary activity. Our dust
detections are sensitivity limited, therefore our measured activity rate is
necessarily a lower limit. All comets with small perihelion distances (q < 1.8
AU) are inactive in our survey, and the active comets in our sample are
strongly biased to post-perihelion epochs. We introduce the quantity
epsilon-f-rho, intended to be a thermal emission counterpart to the often
reported A-f-rho, and find that the comets with large perihelion distances
likely have greater dust production rates than other comets in our survey at
3-7 AU from the Sun, indicating a bias in the discovered Jupiter-family comet
population. By examining the orbital history of our survey sample, we suggest
that comets perturbed to smaller perihelion distances in the past 150 yr are
more likely to be active, but more study on this effect is needed.
