# Calculating Revenue per customer for a series of emails over time, which have a normal decay rate

I am analyzing data for an email marketing company.

We have x amount of subscribers that come in at some point in time.

We promote zero or more emails to these individuals daily, for several months.

I want to determine what the "best performing" emails are, as a function of Revenue per subscriber (RPS).

• revenue per subscriber = revenue for that email/number of subscribers that email was sent to

Where I am facing difficulty is factoring the natural decline in revenue that occurs as the email list becomes old. Average RPS declines naturally over time for several reasons (people unsubscribe, people stop opening emails, etc.)

The natural decline means that I cannot simply rank the RPS over time and look at what values are the highest, because this would not account for the natural decline. i.e. and RPS of 100 is much more valuable when the list is 6 months old, than when it is 1 week old.

Hopefully that makes sense, and I apologize if this question is simple. I am pretty new to statistics and data analysis, so bear with me.

I would appreciate if you could tell me how to arrive at an answer (what are the best performing emails) as opposed to just giving me a straight answer.