Sample:
            Any part of the population is called a sample. A study of the sample enables us to make some decisions about the properties of the population. The number of units included in the sample is called the size of the sample and is denoted by. A good sample is that one which speaks about the qualities of the population. A sample study leads us to make some inferences about the population measures. This process is called sampling.

Parameter and Statistic:
            Any measure of the population is called parameter and the word statistic is used for any value calculated from the sample. The population mean is a parameter and the sample mean is a statistic. The sample mean is used to estimate the population mean. Similarly the population variance is a parameter and the sample variance is a statistic. In general the symbol is used for a parameter and the symbol  is used for a statistic. The value of the parameter is mostly unknown and the sample statistic is used to make some inferences about the unknown parameter.

Sampling Fraction:
            If the size of the population is and the size of the sample is, the ratio  is called the sampling fraction. If , the ratio . It means that on the averageunits of the population will be represented by one unit in the sample. If the sampling fraction  is multiplied with, we get the sampling fraction in percentage form. Thus, . It means of the population is included in the sample.