I want to run PCA on a data set that will be aggregated by country. Should I aggregate the data before or after I standardize the data, and why?
Data should be standardized for calculating principal components. If data is standardized before aggregating by country, the resultant data after aggregation will no longer will have mean 0 and standard deviation 1, hence, the results of PCA will be erroneous.
Therefore, data should be aggregated and then standardized in order to obtain correct principal components of the data.