Which value would you like to know, the sample mean, Y ̄, or the population mean μ? Why?
The population mean, μ, is the underlying value of interest. The sample mean, Y ̄ , is based on μ, but also contains sampling error. So to understand the future, that is, to forecast the value of the variable, the forecast is based on the underlying stable μ, which describes the stable aspects of the underlying system.