Question

In advance of the $1936$Presidential Election, a magazine titled Literary Digest released the results of an opinion poll predicting that the republican candidate Alf Landon would win by a large margin. The magazine sent post cards to approximately $10,000,000$prospective voters. These prospective voters were selected from the subscription list of the magazine, from automobile registration lists, from phone lists, and from club membership lists. Approximately $2,300,000$people returned the postcards.

a. Think about the state of the United States in $1936$. Explain why a sample chosen from magazine subscription lists, automobile registration lists, phone books, and club membership lists was not representative of the population of the United States at that time.

b. What effect does the low response rate have on the reliability of the sample?

c. Are these problems examples of sampling error or non sampling error?

d. During the same year, George Gallup conducted his own poll of $30,000$prospective voters. These researchers used a method they called "quota sampling" to obtain survey answers from specific subsets of the population. Quota sampling is an example of which sampling method described in this module?

Short answer

(a) The chosen sample is not representative of the population of the United States.

(b) Due to low responses to the respondents, there are huge chances that the conclusion of the study will go wrong.

(c) Since the respondents of the selected sample refused to respond to the survey, hence it is a sampling error.

(d) Quota sampling is an example of a non-probabilistic version.

Step-by-step solution

Definition 

Quota sampling is a sampling approach that collects data from a homogeneous set of people.

Non-probabilistic sampling method is each subgroup is included and incorporated in the final sample.

Explanation (part a)

The lists selected to choose the sample from cover the rich and the affluent people of US, in no manner does it represent the average, middle class or poor citizens of US who are the prospective voters too, hence these lists fail to be the correct representation of the population of US at that time.

Explanation (part b)

The low response rate fails to give the true picture of the preference of the public for voting. There may be some who do not want any of the available options, there may be people who would prefer another party, or there may be many other scenarios involved which would be clearly visible only when all the people respond to the survey and provide the true picture of the polls. Hence low response rate leads to less reliable results.

Explanation (part c)

The error caused due to the non-response of the subjects in the sample is not caused due to some flaw in the sampling design. Hence it cannot be considered to be a sampling error. Since it is caused by other reasons or other influencing factors, the error of no-response is considered to be a non-sampling error.

Explanation (part d)

In this method, each subgroup is included and incorporated in the final sample. The proportions of all subgroups with respect to the population are considered while selecting the final sample, thus even if the subgroup is small or negligible, it gets shown in the final sample. That is the reason why it is considered to be a non-probabilistic sampling technique.