Let's say I have a database to track my customers.
On any given day, a customer is active or non-active, and I can discover this in real-time by querying the database. So, right now, I can easily find out that I have X active customers.
What I can't do is discover how many customers I had on July 5, 2014, for example. There is no way to reconstruct a particular point in time, to find out this data point for any given moment in the past. (Not uncommon — it would be fairly rare, actually, for a database package to do this. Databases generally know about their state at the current moment only.)
To solve this, I create a job that queries the database every night at midnight, captures this data point, then writes to a file with a date. So, I end up with a series of timestamped numbers which represent this particular point of data at a regular interval over a period of time:
2014-07-05: 82
2014-07-06: 84
2014-07-07: 81
2014-07-08: 76
[etc.]
These numbers represent this data point, trending over time. This is a historical, indisputable record of a single data point at a single, specified moment in time.
Is there a name for this particular aspect/paradigm on this number? What would I call this data set, collectively?
Best Answer
You could call it a time series or a time-series variable (though most people neglect the hyphen). You could also use the more general term of longitudinal data; time-series data entail one type of longitudinal data. (If the measurements were taken at only two points, this would hardly constitute a time series and would have to be analyzed with other methods besides time-series methods.)
Analysts typically seek to answer two types of questions using time-series data. One is "to what extent can we explain the changes that occur?" -- as by creating a model of that single variable. ARIMA modeling is an example of such an approach. A second is, "To what extent can we explain changes in one such variable as a function of one or more others?"
(A bit more on terminology: while the series of numbers is a variable, the term "data point" applies only to any individual number, such as the 82. And then "data set" would usually refer to a group of such variables that might be analyzed together.)