How do I know if I have One of population or two?

Hi Youness!

Thanks for reaching out!

Generally, we're dealing with one population when we're interested in estimating a parameter (like the mean) of a single group or dataset. On the other hand, we're considering two populations when we're comparing two different groups or datasets. 

Trying to calculate the number of shoes likely to be sold based on historical data, means that we are dealing with one population. Here, the population is the historical sales data of the shoes. We are analyzing this single dataset to estimate a parameter (in this case, the future number of shoes likely to be sold), which is a characteristic of this one population.

If we want to determine whether two shops are selling the same amount of shoes, we are dealing with two populations. Each shop represents a different population, and we're interested in comparing a parameter (the number of shoes sold) between these two groups.

Hope this answers your question.

Best,

Ivan