# When to use current US\$vs. constant 2015 US\$ vs. PPP?

Given that I am interested in comparing various countries by their GDP per capita - which way of calculating this metric should I use? Namely, when to use current US\$, when to use constant 2015 (or other year) US\$, and when PPP (current or constant international \$one)? What are pros and cons of each approach and which one is most popular? I have seen this question: World bank data - GDP per capita current US\$ vs constant 2005 US\\$ and I understand how these different ways of calculating statistic work. But I still find it not entirely clear when to use one over the other?