# Difference between longitudinal data and panel data

When trying to grasp the terminology behind some time-series concepts, I have came across Rob Hyndman's article which states the following

Econometricians use “panel data” while statisticians use “longitudinal data” to refer to collections of time series.

After searching (check the search done bellow), I end up with the idea that the concepts are the same.

Is the fields that the terms are being used the only difference between longitudinal data and panel data?

## Search

1. Longitudinal data:

1.1. This blog entry defines it as

Longitudinal data typically arise from collecting a few observations over time from many sources, such as a few blood pressure measurements from many people.

Then longitudinal analysis is the study of collections of variables; in most applications the variables are strongly associated. We can associate each time point with a separate variable, in the spirit of the original definition of the term variable.

1. Panel:

2.1. This article it looks like Panel Analysis is the same as Longitudinal Analysis

Longitudinal or panel data analysis refers to the statistical analysis of pooled data which consists of a cross‐section of units (e.g., countries, firms, households, individuals) for which there exist repeated observations over time.

2.2. This Wikipedia page goes within the same line as the article in 2.1.

In statistics and econometrics, panel data and longitudinal data are both multi-dimensional data involving measurements over time.

2.3. This Quora answer use the terms interchangeably

longitudinal or panel data, observations of multiple phenomena over multiple time periods for the same data units; involves repeated observations of the same variables (e.g., people) over periods of time; can prove cause and effect, but time-consuming and expensive

• @ G. Peres I appreciate your well researched question! In extension to @markowitz's response I would add that the focus should be on the purpose for which the data were collected and the results sought. I would leave the rest to semantics. Commented Dec 9, 2020 at 17:44