Sample Survey Designs: Basic Concept (Sector 4)

1. Probability and Non-Probability Sampling
   1. The technique of selecting a sample is of fundamental importance in sampling theory and usually depends upon the nature of the investigation. The sampling procedures which are commonly used may be broadly classified as under the following heads:
      1.                                                               i.      Probability sampling
      2.                                                             ii.      Non-probability sampling

1.         i.            Probability Sampling
   1. This is the method of selecting samples according to certain laws of probability in which each unit of the population has some definite probability of being selected in the sample. It is to be noted here that there are number of samples of specified types S₁, S₂, …, S_k that can be formed by grouping units of a given population, and each possible sample S_i has, assigned to it, a known probability of selection p_i. A clear specification of all possible samples of a given type along with their corresponding probabilities of selection is said to constitute a sampling design.

1.       ii.            Non-Probability Sampling
   1. This is the method of selecting samples, in which the choice of selection of sampling units depends entirely on the discretion or judgement of the sampler. This method is also sometimes called purposive or judgemental sampling. In this procedure, the sampler inspects the entire population and selects a sample of typical units which he considers close to the average of the population.
   2. This sampling method is mainly used for opinion surveys, but cannot be recommended doe general use as it is subject to the drawbacks of prejudice and bias of the sampler. However, if the sampler is experienced and an expert, it is possible that judgemental sampling may yield useful results. It, however, suffers from serious defect that it is not possible to compute the degree of precision of the estimate from the sample values.

1. Sampling Unit
   1. A sampling unit has already been defined in the previous sector, but it needs further description as it forms the basis of sampling procedure. These units may be natural units of the population such as individuals in a locality, or natural aggregates of such units such as family, or they may be artificial units such as a farm, etc. Before selecting the sample, the population must be divided in to parts which are distinct, unambiguous and non-overlapping, such that every element (smallest component part in which a population can be divided) of the population belongs to one and only one sampling unit. Since the collection of all sampling units of a specified type constitutes a population occurs just one in sampling unit. Otherwise, some of the elements will not be included in any sample. For example, if the sampling unit is a family, it should be so defined that an individual does not belong to two different families not should it leave out any individual belonging to it.

1. Sampling Frame
   1. As define earlier, a complete list of sampling units which represents the population to be covered is called the sampling frame popularly, known as frame. The construction of a sampling frame is sometimes one of the major practical problems. Generally, it is assumed that a frame is perfect if it is exhaustive, complete, and up-to-date in respect of sampling units and characters structures. So the frame should always be made up-to-date and free from errors of omission and duplication of sampling units. Different aspects of sampling frames have been discussed by Mahalanobis (1944), Yates (1960), Seal (1962), Hansesn, Hurwitz and Jabine (1963), Singh (1978) and others. A sampling frame is subject to several types of defect which may be broadly classified as follows:
      1.                                                               i.      A frame may be incomplete. When some sampling units of the population are either completely omitted or included more than once.
      2.                                                             ii.      A frame may be inaccurate. When some of the sampling units of the population are listed inaccurately or some units which do not actually exist are included in the list.
      3.                                                           iii.      A frame may be inadequate. When it does not include all classes of the population which are to be taken in the survey.
      4.                                                           iv.      A frame may be out-of-date. When it has not been updated according to the exigencies of the occasion, although it was accurate, complete and adequate at the time of construction.
   2. Lists which have been routinely collected for some purpose are generally found to be incomplete and inaccurate and often contain unknown amounts of duplication. Such lists should be carefully examined and scrutinised to ensure that they are free from all such defects. If they are not up-to-date, they should be made so. A good frame is difficult to construct, but experience always helps in its construction.

    

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