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 .