What is the terminology for the length of time an individual data point represents? I'm having trouble conceptualizing this or putting it into word(s). It has been bothering me off and on for months now.
As an example (financial, as that's all I know), let's say that Sarah wants to measure her financial success using two main metrics on a quarterly basis: Gross Income and Net Worth
For both of these metrics, the periodicity would be "Quarterly". A key difference between the metrics however, is that Quarterly Gross Income measures the sum of Sarah's income over the span of a Financial Quarter, whereas "Net Worth" is what I would call a "Point in Time" observation that just so happens to be observed on a quarterly basis.
What is the terminology or concept around this "duration of measurement" that I'm missing? Or am I just way off in my way of thinking around this?
Thanks!
 A: As per my knowledge, there is no specific term for the periodicity for Net Worth. The net worth is like a continuous function and could not be attributed to any specific period. Even if we calculate if in quarter, that information will be applicable for that specific time on that day in that quarter. 
For example, let's say Sarah's date of birth is February 10, 1990.  We could say that she was 19 years old in January, 2020. However, we could say that neither she was 19 years old in the quarter Jan-Mar, 2020 nor she was 20 years old in the quarter Jan-Mar, 2020. We couldn't say that she was 19 years on January 1, 2020. 
Whenever, we mention the net worth for a specific quarter as per your example, we could say "net worth at the end of the quarter" or "net worth at the beginning of the quarter".
I think I have to search through the literature to give you a more convincing answer than this. I'll keep you posted, unless someone else comes up with the answer. 
A: I would call this "resolution" or more specifically "time resolution": you are sampling a continuous function at discrete time points. You could also refer to a "sampling interval" or "sampling frequency". Your data don't really represent a time interval, they are just sampled at particular points in time; the numbers would change if you shifted the samples by 1 month, but the time resolution would stay constant.
If you were to generate a plot of these data, the resulting trace would represent a discrete approximation of Sarah's net worth at a quarterly resolution.
A: In Economics, there is an standard distinction between so called "flow variables" and "stock variables". Flow variables are those which do not have meaning for a point in time, but rather can be thought of as integrals. Income or production of a certain good are examples. Stock variables have meaning at a point in time: interest rates, prices, etc.
