The population is difficult to cover because it involves a lot of cost on procedure. Further, whole population is difficult to cover and it is infeasible to collect data on entire population. For example we want to check the quality of rice in production unit, we could n't test all grains of rice to check the quality rather than we take a portion of rice grains for quality check. Here, if the whole lot of a rice grains are taken then first drawback would be that it will be costly and second it is infeasible to collect data for whole lot of a rice grains.
2)
Teacher's union is planning to calculate the average salary for only high school teachers in a country and salaries for other teachers are not considered. So, from teachers population a sample of high school teacher in a country is selected. Thus, the calculated average will be sample statistic.
