#### ps_ricky_son

##### Member

Grateful if you could help the below concepts again.

1. In the variance-variance matrix, there is a concept of positive semi-definitive which the Factor model qualifies, what is the concept about? quite confused with the transpose of vector/ vector mentioned in the book

2. the GARCH model is said to perform a number of excellent/very good/good jobs regarding the explanation on volatility, what in fact account for the differences among those descriptions? Can I say excellenet job means greater explanatory power?

3. the AR model's auto correlation is said to be oscillate, why is it so?

4. in the calculation of long run variance, sometimes we need to multiply the answer with the square of the frequency in a year to derive the annualized volatility. So, what is the unit of the long run variance measurement then? (i.e. is the original answer before multiply measured in days or according to what the question specifies? )

Many thanks again.