I'm not sure how to look up and read about this problem, and I'm hoping you might be able to point me in the right direction. In essence, this question is about whether or not I should purchase another unit this year:
Given that I have sold 3 of item x this year, what is the probability that I will sell a 4th?
I think this is related to a manufacturing question of this variety:
Given that the machine has produced 3 units without failure what is the probability that it will produce a 4th unit without failure?
In a less simplified version of this question, you might take into account that sales is a time series (that unit 1 was sold at time 0, unit 2 was sold at time 2 and unit 3 was sold at time 2.5), and attempt to predict the probability that unit 4 will be sold within time 5 - but I'm not sure I have the statistical foundation to understand how to do that quite yet.
What is this type of problem called, and what should I focus on learning to be able to answer it?