As someone new to statistics, I had a debate with my supervisor today on the definition of population and sample in the following case.
Suppose we track all new users (1,000 in total) that used an app during a period of time, and want to look at the completion rate at each milestone step. The data looks like this below:
- Started Step 1: 1,000
- Finished Step 1: 800 (completion rate = 80%)
- Started Step 2: 700
- Finished Step 2: 600 (complete rate = 85.7%)
My supervisor's opinion is that the 1,000 new users are the whole population, because they are all the new users that used the app during the target period. Their completion rates are the true rates.
On the other hand, I think these 1,000 users are only a sample, while the whole population should be all the potential new users, whose size is unknown. And, we can only use these completion rates to estimate the true rates of the unknown population.
So the 1st question is, who is correct?
Then, what makes me more confused is that my supervisor also asks me to use a sample size calculator to make sure the population size of each step is large enough, so that the completion rates are statistically significant.
So my 2nd question is, if we treat the 1000 new users as the population, is it even necessary to check the population size? Is it not what it is?
And also a 3rd question - assuming I'm correct in saying these 1000 new users are just a sample, can I loosely say that the sample size of each step is large enough, because the numbers are much larger than 30? (I read from some materials that 30 should be the minimum size by rule of thumb).