Elementary Statistics (1+1)

3.1.13 Merits and limitations of sampling

Merits. The sampling technique has the following merits over the complete enumeration survey:

1. Less time consuming. Since the sample is a study of a part of the population, considerable time and labour are saved when a sample survey is carried out.  For these reasons a sample provides more timely data in practice then a census.

2. Less cost. Although the amount of effort and expense involved in collecting information is always greater per unit of the sample than a complete census, the total financial burden of a sample survey is generally less than that of a complete census. This is because of the fact that in sampling, we study only a part of population and the total expense of collecting data is less than that required when the census method is adopted. This is a great advantage particularly in an underdeveloped economy where much of the information would be difficult to collect by the census method for lack of adequate resources.

3. More reliable results . Although sampling involves certain inaccuracies owing to sampling errors, the result obtained is generally more reliable than obtained from a complete count. There are several reasons for this. First, it is always possible to determine the extent of sampling errors. Secondly, other types of errors to which a survey is subject, such as inaccuracy of information, incompleteness of returns, etc., are likely to be more serious in a complete census than in sample survey. This is because more effective precautions can be taken in a sample survey to ensure that information is accurate and complete. For these reasons not only may the total error be expected to be smaller in a sample survey but sample result can also be used with a greater degree of confidence because of our knowledge of the probable size of error. Thirdly, it is possible to avail the services of experts and to impart thorough training to the investigators in a sample survey which further reduces the possibility of errors. Follow-up work can also be undertaken much more effectively in the sampling method. Indeed, even a complete census can only be tested for accuracy.

4. More detailed information. Since sampling saves time and money, it is possible to collect more detailed information in a sample survey. For example, if the population consists of 1,000 persons in a survey of the consumption pattern of the people, the two alternative techniques available are as follows:

(a) We may collect the necessary data from each one of the 1,000 people through a questionnaire containing, say, 100 questions (census method), or

(b) We may take a sample of 100 persons (i.e., 10% of population) and prepare questionnaire containing as many as 10 questions. The expenses involved in the latter case would almost be the same as in the former but it will enable nine times more information to be obtained.

5. Sampling method is the only method that can, be used in certain cases. There are some cases in which the census method is inapplicable and the only practicable means is provided by the sample method. For example, if one is interested in testing the breaking strength of chalks manufactured in a factory under the census method all the chalks would be broken in the process of testing. Hence, census method is impracticable and resort must be had to the sample method. Similarly, if the producer wants to find out whether the tensile strength of a lot of steel wires meets the specified standard, he must resort to sample method because census would mean complete destruction of all the wires. Also if the population under investigation is infinite, sampling is the only possible solution.

6. The sample method is often used to judge the accuracy of the information obtained on a census basis. For example, in the population census which is conducted very often (10 years in our country) the field officers employ the sample method to determine the accuracy of information obtained by the enumerators on the census basis.

Limitations. Despite various advantages, sampling is not completely free form limitations. Some of the difficulties involved in sampling are stated below:

1. A sample survey must be carefully planned and executed, otherwise the results obtained may be inaccurate and misleading. Of course, serious errors may arise in sampling, if the sampling procedure is not perfect.

2. Sampling generally require the service of experts, if only for consultation purposes. In the absence of qualified and experienced persons, the information obtained from sample surveys cannot be relied upon.

3. At times the sampling plan may be so complicated which may require more time, labour and money than census. This is so if the size of the sample is a large proportion of the total population and if complicated weighted procedures are used. With each additional complication in the survey, the chances of error multiply and greater care has to be taken which, in turn, means more time and labour.

4. If the information is required for each and every unit in the domain of study, a complete enumeration survey is necessary.