Sampling is defined as the selection of some part of an aggregate or totality on the basis of which an inference is made on the aggregate or totality. It is the technique of selecting a number of units for the study from a defined study population. It is an integral part of data collection process. For example, a pathologist takes a few drops of blood and tests for any change in blood of whole body; a quality controller takes a few items and decides whether the lot is in accordance with the desired specification or not.
Even in case of complete enumeration, sampling is preferred in order to save time, expense, manpower etc. Sampling is the technique of selecting sample from the population such that all the characteristics of the population are contained in the sample. In statistical study, data are collected from population under study. The population consists of very large number of units, sometimes infinitely many. In many situations it is not possible to include all the units. So, a few units are selected from the population, which are known as samples, and the method of selecting sample is called sampling.
Needs of Sampling
Sampling is carried out because of the following reasons:
- Sampling saves time and money. Sample study is less expensive than census and gives faster result.
- It is the only choice if the test involves the destruction of items under investigation. Sample study prevents the items from destruction if items are of destructive nature.
- It is the only way for infinite population. For infinite population, complete enumeration is not possible, so sample method is only the method of collecting information.
- It enables the estimation of sampling error. The sample estimate gives the error from the population.
- It enables more accurate result. Sampling conducted by well-trained and experienced investigators gives accurate result.
Advantages of Sampling
- It saves time not only while collecting data but also while processing.
- It saves cost.
- It is feasible to collect more detailed information.
- The result obtained is generally more reliable.
Disadvantages of Sampling
- If sampling procedure is not perfect, then errors may be introduced.
- It require service of experts for proper planning and execution.
- If information of each and every unit is required, then it cannot be adopted.