Sampling theory

Sample means a representative of a population. That means a sample posses almost all the qualities of a population. 
It is difficult to collect information from the total population. Instead, a smaller subgroup of the target population or a sample is selected for the purpose of study. Sampling is the strategy of selecting a smaller section of the population that will accurately represent the patterns of the target population at large. 

hope you got an idea ........Best wshs..........
 

https://en.wikipedia.org/wiki/Statistical_sampling nice article on sampling I think it would cover everything you want to know