Does anyone know what is the difference in a regression between and ? One is called the unconditional one the other conditional. So far I can get it. But conditional on what exactly? Does it mean that refers to the expected value of in the (total) population, while to a chosen sample and we treat X as fixed? But if we had no X, how would be obtain otherwise? And why would we fix X? I am not exactly sure where their difference lies.. Please help.. I know it's a basic question but it confuses me.