We built a segment that has a couple conditions (drives and training), and we want to measure unique visitors, spanning multiple months (e.g. September - December 2018). It is possible for a drive or training to occur in 1 month, then driving or training to occur in subsequent months.
When we run the report each month, we expand the workspace date range and notice that historical data has changed. Once segment criteria is met, does the unique visitor count get applied to the original month it was started or the latest month when both criteria has been met? If the original month, this may explain why historical numbers are changing.
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I try do explain it and hope I ubderstand the question. assuming you are looking for all visitors (your segment) who had both an event A and B.
if user X has event A in October and event B in December, he will appear (match the segment) only after the second event happened. that means the December report will include the visitor for the first time. but looking back (history) the user would appear in the same reporting for "unique visitors" in October as well (for the first time), since he had the first event in this month.
If you are using the mentioned "visitor segment", historical data would change. according to the example above the december report would have other numbers for at least October ...
you could create a new segment "HIT" with your original visitor segment in it and add a new condition for event B. this would return only the hits for event B while the visitors must have event A. using this segment would not affect historical data because the user in the example would only appear for the month where event B occured.