Uses Gamma/Gompertz/NBD model parameters and a customer's past transaction behavior to return the probability that they are still alive at the end of the calibration period.
ggnbd.PAlive(params, x, t.x, T.cal)Gamma/Gompertz/NBD parameters - a vector with r, alpha,
b, s and beta in that order.
number of repeat transactions in the calibration period T.cal, or a vector of calibration period frequencies.
recency, i.e. length between first and last transaction during calibration period.
length of calibration period, or a vector of calibration period lengths.
Probability that the customer is still alive at the end of the calibration period.