Multi Stage Sampling
This is a modified form of cluster sampling. While in cluster sampling all the units in a selected cluster constitute the sample in multistage sampling the sample units are selected in two or three or four stages. In this system the universe is first divides into first stage sample units from which the sample is selected. The selected first stages samples are then sub divided into second stages units from which another sample is selected. Third stages and fourth stages sampling is done in the same meaner if necessary. Thus for an urban survey a sample of towns may be taken first and then for each of the selected town a sub sample of households may be taken and then if need be from each of the selected household a third stages sample of individuals may be obtained.
If clusters happen to be some geographic subdivisions in that case sampling all the units in a selected cluster constitute the sample in multistage sampling the samples units are selected in two or three or four stages. In this system the universe is first divided into are then sub divided into second stage units from which another sample is selected. Third stage and fourth stage sampling is done it the same manner if necessary. Thus for an urban survey a simple of towns may be taken first and then for each of the selected town a sub sample of households may be taken and then if need be from each of the selected household a third stage sample individuals may be obtained.
There are two merit of this sampling design viz,
a.It is easier to administer than most single stage design mainly because of the fact that sampling frame under multi stage sampling is developed in partial units.
b.A large number of unit scan be sampled for a given cost under multistage sampling because of sequential clustering whereas this is not possible in most of the simple designs.
Advantages of Probability Sampling
a.Probability sampling is the only sampling methods that provides essentially unbiased estimates having measurable precision. If the investigator requires this level of objectivity then some variant of probability is essential/.
b.A second advantage is that probability sampling permits the researcher to evaluate in quantitative terms the relative efficiency of alternate sampling techniques in a given situation usually this is not possible in non probability sampling.
c.A third major advantage is that probability sampling requires relatively little universe knowledge: essentially only two things need to be known.
a. A way of identifying each universe element uniquely and
b. The total number of universe element.
If in area sampling a correspondence can be established between area units and universe items and area maps are available then probability sampling can be used. In contrast non probability sampling methods require substantial information.
Disadvantages of Probability Sampling
a. It is relatively expensive than non probability sampling.
The methods of selection in many case can be complex and time consuming. Specially in the cases of marketing research