https://www.youtube.com/watch?v=cZssJ1SAESs&list=PLEUKy_nwlzwFbxj6ilK2XXHJqBXMcTOOc&index=5
================================================================================
Subject: each person
Factor or Concept
Characteristics which subject has
Variable: Factor
Measurement
By using rule, data is represented by symbol or number
Data
Collection of symbols or numbers
================================================================================
================================================================================
4 variables
================================================================================
================================================================================
================================================================================
Cross-sectional data
data which is obtained from one time
================================================================================
Longitudinal data
Time series data
data which is obtained from continuous time
ex) stock prices, GDP variation from 2010 to 2014
================================================================================