  We describe a method of estimating the abundance of short-period extrasolar
planets based on the results of a photometric survey for planetary transits. We
apply the method to a 21-night survey with the 2.5m Isaac Newton Telescope of
\~32000 stars in a ~0.5 deg by 0.5 deg square field including the open cluster
NGC 7789. From the colour-magnitude diagram we estimate the mass and radius of
each star by comparison with the cluster main sequence. We search for injected
synthetic transits throughout the lightcurve of each star in order to determine
their recovery rate, and thus calculate the expected number of transit
detections and false alarms in the survey. We take proper account of the
photometric accuracy, time sampling of the observations and criteria
(signal-to-noise and number of transits) adopted for transit detection.
Assuming that none of the transit candidates found in the survey will be
confirmed as real planets, we place conservative upper limits on the abundance
of planets as a function of planet radius, orbital period and spectral type.
