Sampling is mainly divided into two types; probability or random sampling and non probability or non random sampling. In other words, Sampling methods are broadly classified into probability or random sampling and non probability or non random sampling. Let us discuss them one by one:
1) Probability or random sampling: Under the probability or random sampling, each and every unit of the universe has an equal chance of being selected. It is the best technique based on the law of statistical regularity.
The probability or random sampling is of four types:
i) Simple or nonrestrictive random sampling: in this method of sampling each and every unit of the universe has an equal chance of being selected as sample. This method is further divided in two parts.
a) Simple random sampling without replacement: if the unit selected in any draw is not replaced in the population before making the next draw then it is called simple random sampling without replacement.
b) Simple random sampling with replacement: if the unit selected in any draw is replaced in the population before making the next draw then it is called simple random sampling with replacement.
Also Read: meaning and principles of sampling
Methods of selecting simple random sample:
a) Lottery method: in this method the slips are put in a bag and shuffled and then drawn one by one.
b) Tippet’s random number: it consists 10400 four digit number. 10400*4= 41600 selected at random.
c) Fisher and Yates: it consist 15000 digits.
d) Kendall’s random number: it consist 100000 digits grouped into 25000*4.
e) There may be grid system for selecting a sample.
f) Sample may be selected from arranged list.
ii) Stratified random sampling: under this method of the probability or random sampling, population having same characteristics is divided into groups and sub groups. This grouping or sub grouping is called ‘strata’. Each unit in each stratum is homogenous. The various strata are known as stratifying factor like age, sex income etc. it is different from simple random sampling because in it unit are selected randomly from entire population while in stratified random sampling the units are selected from the each strata.
iii) Systematic sampling: it is a little variation of simple random sampling as in this first unit is selected at random and other units are automatically selected in a definite sequence. For example name of persons in telephone dictionary. Its formula is
K= sample interval
N= universe size
n= sample size
iv) Multistage or cluster sampling: in this case of the probability or random sampling, the total population is divided into clusters. For example population in a city is divided into n blocks. Clusters are primary units and units within the clusters are termed as secondary units. It is known as multistage cluster. For example if we are interested in obtaining the sample of different colleges, then first we will select the state, then district, then we select the particular colleges. Thus each stage results in reduction of sample.
The above the two types of the probability or random sampling.
Let us discuss the non probability sampling.
2) Non probability sampling or non random sampling: under this kind of sampling each and every unit of the universe has not an equal chance of being selected.
i) Judgment sampling: under this method of the non probability sampling or non random sampling, desired numbers of sample units are purposely or deliberately selected by the researcher. So the important items representing the true characteristics of the population are deliberately selected. In this case there are the chances of personal biasness convenience and the time the researcher has. For example if the researcher wants to show the employed persons are more than the unemployed persons in the city then he will collect the data mostly from the employed persons.
ii) Convenience sampling: this method is also called chunk. Chunk means that fraction of population being investigated neither by probability nor by judgment but on the basis of the convenience. A sample is drawn from readily available list on the convenience of the researcher.
iii) Quota sampling: this method of the non probability sampling or non random sampling depends upon personal judgment. Quota is set up according to some specified characteristics like how many students of equal grade.
These are the various types of non probability sampling or non random sampling.
I hope you have understood the various types or methods of sampling.