A sample is a subset of a population. Usually it is impossible to test an entire population so tests are done on a sample of that population. These samples can be selected so that they are representative of the population in which cases the sample will have weights, strata, and clusters. But usually people use random samples. So it's not that the line is different, it's that the line comes from different data. In stats we have formulas that allow a sample to represent a population, if you have the entire population (again unlikely), you wouldn't need to use this sample formulas, only the population formulas.
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The sample regression function is a statistical approximation to the population regression function.
Sampling Error
population is the number of citizens living in a defined geographical area. Sample is a number taken from the population being the sample to research for a topic about the populations' behavior or habit, etc.
standard error
1. PRF is based on population data as a whole, SRF is based on Sample data 2. We can draw only one PRF line from a given population. But we can Draw one SRF for one sample from that population. 3. PRF may exist only in our conception and imagination. 4. PRF curve or line is the locus of the conditional mean/ expectation of the independent variable Y for the fixed variable X in a sample data. SRF shows the estimated relation between dependent variable Y and explanatory variable X in a sample.