Data or Data Set
Data is a set of measurements of some qualitative aspect or quantitative aspect. If we record earnings of five persons in a city, say 100, 200, 200, 500, 1000 (in Rs) then those figures will be our data set or data.
Unit is an individual about which data is to be collected. e.g Person.
Observation is individual measurement in a data set. e.g Rs. 700
Quantitative data is data which has numerical value e.g. the above data. Another example is marks obtained by students in a class.
Quantitative data is data which does not have numerical value. It is data which is of descriptive nature. e.g. color of eyes
Population or Universe
The entire collection of objects or persons about which inferences are to be drawn is called as population or universe. e.g If some conclusions are to be drawn about the students in a particular college, all students of that college will comprise the population.
The part of the population selected for the purpose of the study is called as sample. In above case it will be difficult to interview all the students of the college as the total number of students could be in thousands. In such case one would select a few students for interview. Students selected for interview comprise of sample.