Difference between longitudinal data and panel data

Question

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.

1.2. This article defines it as

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.

2. 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

Answer 1

Longitudinal and panel data are synonym. There are only three possibility:

1. cross section: several variable one time only
2. time series: one variable only measured many times
3. panel/longitudinal: several variables measured several times

Moreover note that, at least in econometrics, the fact that we have a panel dataset do not imply that we must use models for panel data. For example a simple VAR (vector autoregression) is a typical time series model and do not represent a panel data technique, even if the dataset would be suitable for.

Answer 2

Longitudinal data refer to repetitive measurements over time. The measurement could be on the same units or otherwise. Panel data are a type of longitudinal data where the observed units are the same. Repeated cross-sections are longitudinal but can hardly be considered panel, at least in econometric modelling.