  To provide criteria in the selection of target events preferable for
planetary lensing follow-up observations, we investigate the variation of the
probability of detecting planetary signals depending on the observables of the
lensing magnification and source brightness. In estimating the probability, we
consider variation of the photometric precision by using a quantity defined as
the ratio of the fractional deviation of the planetary perturbation to the
photometric precision. From this investigation, we find consistent result from
previous studies that the probability increases with the increase of the
magnification. The increase rate is boosted at a certain magnification at which
perturbations caused by central caustic begin to occur. We find this boost
occurs at moderate magnifications of $A\lesssim 20$, implying that probability
can be high even for events with moderate magnifications. The probability
increases as the source brightness increases. We find that the probability of
events associated with stars brighter than clump giants is not negligible even
at magnifications as low as $A\sim 5$. In the absence of rare the prime target
of very high-magnification events, we, therefore, recommend to observe events
with brightest source stars and highest magnifications among the alerted
events. Due to the increase of the source size with the increase of the
brightness, however, the probability rapidly drops off beyond a certain
magnification, causing detections of low mass ratio planets ($q\lesssim
10^{-4}$) difficult from the observations of events involved with giant stars
with magnifications $A\gtrsim 70$.
