Stellar populations. I) What is the difference between population and sample?

How many stars are in the Milky Way? The total population of stars in our Galaxy is enormous (about a few hundred billion). We cannot count or study every single star in our Galaxy, hence Astronomers must select relatively small samples of stars (hundreds or thousands) to study our Galaxy.

Milky Way as seen from Paranal. (c) ESO

How to select a sample that is representative of a large population? This is far from trivial; incorrect conclusions can be obtained if there is a bias in the selection. An example is the study of the mean age of the whole population of stars in our Galaxy. Imagine that to do this job you select a sample of stars located in the Galactic halo, far away from the Galactic disk. Halo stars are old (about 10 Gyr), therefore you would incorrectly conclude that stars in our Galaxy are mostly old.

Problems with sample selection can also affect our daily life. Think about polls for a Presidential election, like the one we're having in Brazil. Poll companies cannot interview the population of all Brazilian voters, they have to select a sample that is not necessarily representative of the entire population. Indeed, the results of the first round were surprising, candidate Aécio Neves was the runner-up, gathering far more votes than those estimated from the polls. He will face current President Dilma in a second round, on October 26, 2014.