Suppose I have a record of every purchase of food made in 2019 and 2020 and I want to answer the question: how much did I spend on food in March/April/May in those two years (to see how much I'm saving on food while on lockdown).
I can easily just sum the expenditures in that exact interval, but it would be subject to error from purchases outside the interval. For instance, suppose on 2/28/2019, I bought a gallon of olive oil and so didn't need any during the following three months. That purchase should be "charged" to the 2019 interval in some sense, but wouldn't be, because it was slightly outside the interval.
Likewise, a big purchase within the interval would continue to be consumed outside the interval and should not be charged to the interval.
I'd like a way to measure expenditures within the interval, but also outside of it in some proportion of distance in time from the interval, to estimate the true expenses. What would be a technique for this?
