Target versus sampled population I'm having a difficult time understanding the difference between target population and sampled population using stratified sampling. For example, say I want to run some statistics on university students in the UK -- in particular, I want to know specifically about the first-year students. However, let's say the  male/female ratio is 70/30, so I want to stratify the samples and take 1000 students from each gender group.
If the target population is defined as the "whole group of interest," does that mean all university students in the UK? If so, then I'm inclined to say the sampled population are university first-years, and the sampling frame is the 2000 students (1000 male/1000 female) chosen from my strata. However, I keep going back and thinking the target population is actually the university first-years in the UK, not the entire university population (first, second, third, fourth years).
Any guidance or tips would be appreciated.
 A: The target population of a survey is the population you wish to study. The sampled population is the population which you are able to observe in a sample. In an ideal world the target population and the sampled population would be the same, but often they are different.
For example, governments around the world survey active businesses to determine estimates for labour force, productivity, innovation, capital expenditure, etc. Typically this is done by maintaining a register of businesses and sampling from this register. However, there is a gap of weeks/months between when this sample was created to when the survey goes to field. This means there are businesses who have folded after being selected for the survey (ie these entities are no longer in the target population because they have become inactive) and there are new businesses who have started and cannot be surveyed (ie these entities are not in the survey population even though they are active and in the target population). This difference needs to be accounted for, or else we would never be able to include new businesses in these economic measures.
In your case I would keep in mind:


*

*Your target population is theoretical, your sample population is practical, based on the frame you have access to or are able to construct.

*A target population should be more specific than what you've described. Specify exactly the units you mean: for example, full time students studying courses at universities located in the UK on 1st September 2018, excluding those studying by distance not residing in the UK. Note that the specification should be CUTE: content (who the survey is about), units (who you are approaching), time (when the survey is about), extent (where the survey is about)

*The survey and target populations refer to the populations for the entire survey, not individual questions. If you want to ask general questions to all university students, but have some more specific questions for first year students then your survey and target populations are both still all university students, but you may want to control how many first years you survey through stratification to achieve the required accuracy for those questions that specifically concern them. If however you are only interested in first year students but you're sampling other students too, then your target population is just a subset of your sample population and you should try to fix this.
