The choice of an appropriate sampling design is of paramount importance in the execution of a sample survey. Generally, there are two types of sampling techniques; Probability sampling and Non probability sampling. Probability sampling includes the random selection of samples. Simple random sampling, systematic sampling, stratified sampling are the types of random selection. Probability sampling is more complex and time consuming. Non-probability sampling on the other hand, provides a fast, easy and inexpensive way of selecting a sample not based on random selection. However, the statistician must ensure the reliability and representativeness of the sample.
3.1- Probability Sampling Techniques
- Simple Random sampling- Using this technique, each member of a population has an equal and independent chance of being selected. Personal bias of the investigator cannot influence the selection. It should be noted that the word random means the selection process in which the chance only determines which item should be included in the sample. Stratified sampling- Stratified sampling technique is used to represent each segment of the population in the sample. The method consists in dividing the whole population into smaller groups and then appropriate numbers of units are to be selected from each group using random selection.
- Systematic Sampling – This is formed by selecting one unit at random and then selecting additional units at evenly spaced interval. Thus this method is properly used in those cases where complete list of units is available. The first item is selected at random and subsequent items are selected by taking every kth Where
- K=POPULATION SIZE / SAMPLE SIZE
3.2- Non- Probability Sampling Techniques
- Judgment or Purposive Sampling – Under this method, the investigator himself purposively chooses certain items. Hence the selection is distribute and based on own idea of the investigator about the sample units. Here, the chance of inclusion of some items in the sample is very high and others are very low. However for better selection of the item, certain criteria of selection of the items are first laid down and then the investigator is allowed to make the selection of the items.
- Convenience Sampling Method – It is a type of purposive sampling in which the sample units are selected purposively by the investigator to suit his convenience in the matter of location and contacts with the units. The sample are selected neither on the basis of the rules of probability nor on the basis of judgment but on the basis of convenience on the part of the investigator.
- Quota Sampling – It is a purposive sampling in which the whole universe is divided into certain parts and the total sample is allocated among these parts. Each part of the population is assigned to an investigator to be examined by him. This is fixed in advance according to certain specified characteristic such as sex, age, occupation, income group, etc. The investigator is asked to select the required number of units of the sample of his own choice and examine them to get the desired information as quickly as possible. He is also authorized to substitute the new units in the quota if he finds that any unit of the sample so selected is not responding up to the mark..