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I've been trying to resolve an exercise from a textbook where I am faced with the challenge of count different events between sequential stages of an industrial process.

Information related to the process: A test Subject is made to go through a 3 stage process with stages A, B and C respectively the first one being A, second B and finally C; a test Subject may abandon the process at stages A or B and then start again from point A, each time the process takes place a dataset is created with the IDENTIFICATION of the test subject, the TIMESTAMP in which the stage took place and a unique VISIT_CODE During any stage, a test subject may trigger an "ALERT" and this will be recorded with the TIMESTAMP, ALERT_CODE and test subject IDENTIFICATION.

What is to be calculated: I have to create a code in R to count how many ALERTS where generated by the test subjects between stages A and B, between stages B and C and finally how many ALERTS where generated after C. Please note that a test subject may at some point abandon the process to later on start again from point A.

The textbook gives a HINT: "Take a closer look at the stage a test subject is currently to then determine if the ALERT is generated from stage A and before a stage B and also if the test subject triggers an ALERT after stage B and before stage C, but keep in mind that if a test subject abandons at stage A and triggers an ALERT if the TIMESTAMP of that ALERT is smaller than their next attempt at stage A then it should be credited as an ALERT after a stage A"

as another hint, the textbook reveals that the ALERTS after Stage C where only 1 and that it was trigger by test subject W-6 with ALTER_CODE AYUJ-3915716168. The datasets are:

Stage Process