Statistics Assignment Help >> Sample Survey, Bias in Probability Surveys, Non-Probability Based Surveys

Sample Survey

The survey sampling in statistics is done by selecting sample of elements form a given target populations for conducting the survey. The subject of the survey data collection is to contact the members of the sample after they have been selected. Sampling is done in order to reduce the cost and also the amount of work which would have been done if the whole population would have been studied. In case the sample survey measures the entire target population, it is termed as census. A survey very often undertakes different techniques for observation, but survey sampling mostly is done by a questionnaire which measures the characteristic as well as attributes of people.

Survey samples can be broadly classified into probability and non probability samples

In the probability sampling method the member of the target population has a non zero and known probability of getting included in the survey. The probability samples are able to give statistical inferences from the measurement of the target population which can be

1. Unbiased, i.e. the expected value of the sample mean is equal to the value of the population mean. This can be expressed as E (ȳ) =μ, and

2. Have a sampling error which can be measured and is often expressed in margin of error or in sample interval.

Still, biases can be found in the inferences of theses probability based surveys.

The probability based sample survey is done by designing a sample frame, which is list of the target population. In the next stage of selection procedure, units are selected form the sample frame. The selected individual units are then contacted by some method (which often is a mode or data collection method) for collecting the survey.

Bias in probability surveys-

Any survey with biases is undesirable. There are many types of biases that may occur in the sampling process

1. Non-Responsive Bias-when the individual or household who are selected are unable to complete the survey, bias can occur because of this non responsive situation. In response bias, the observe value deviates for the population parameter because of the differences which occur between the non respondents and respondents.

2. Converge Bias- when the population members are not found to appear in the sample frame because of under coverage, the bias is termed as converge bias. Here, deviation is found between the observed values and those in the population parameter.

3. Selection Bias-when some of the units of population has a different probability of selection which is not accounted by the researcher, the bias is termed as selection bias.

Non probability sampling-some survey are carried out by finding a suitable collection of respondents, by whose help the survey is completed. Examples of non probability samples are

1. Judgment samples-the researcher decided which population members are to be included in the sample, by his own judgment. The researcher can also provide for some alternative justification for the representation of the sample.

2. Snowball samples-this method is used when the target population very rare. Members of the target population recruit other members for the survey.

3. Quota samples-the sample is designated for including a definite number of people having certain specified samples. The non probability market research surveys often use this sampling method.

4. Convenience samples-samples are attained on the basis of ease of the availability of the persons for filling the survey.

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