Definition
Time series
A time series is data kept in time order, with each value tied to when it was observed.
What it means
Imagine taking the same kind of snapshot again and again. Each new value joins the end of the record instead of being mixed into a bag. Keeping the time labels lets you notice what changed, what repeated, and what happened first.
Remember it
Keep the clock attached to every value: first, next, next, and so on.
Example
A bean plant is measured every Monday: 8 cm, 13 cm, 19 cm, then 24 cm. Those four measurements form a time series because each height belongs to a successive Monday. Over the recorded period, the height increased by 24 − 8 = 16 cm.
Watch for
Temperatures from eight different cities measured at one moment are a data set, but not a time series of one changing quantity.
Common mix-up
A line graph can display a time series, but the graph is the picture; the time series is the time-ordered data.
Exact meaning
A time series is a sequence of observations indexed by successive times or time periods. The observations may be equally spaced, but the defining feature is that their order in time is preserved.