The article discusses how to test for Purchasing Power Parity (PPP) by using different models. They classify these models into univariate, bivariate, and multivariate types. The main focus is on the real exchange rate and its relationship with relative prices. The researchers found that testing for PPP involves looking at the behavior of the real exchange rate over time. They also highlight the importance of cointegration between the nominal exchange rate and relative prices for PPP to hold in the long run.