The Molecular Diagnostics

are many potential types of errors in survey sampling. According to Groves (1989)[see 1], the survey errors can be divided into two major groups: First, the errors of nonobservation where the sampled elements use only part of the target population, and the second one is the errors of observation, where the listed data deviate from the truth. Some examples of errors of nonobservation can be ascribed to sampling, coverage or nonresponse which is going to be analysed in the later part of this report. On the other hand, examples of errors of observation can be attributed to the interviewer, respondent or method of data collection. Both of our sources of obdurate errors can vigorously affect the accuracy of a survey. However, these errors cannot be eliminated from a survey but their effects can be reduced by careful devotion to an acceptable sampling plan. Some ways to reduce those errors are: callbacks (where the interviewer calls again the nonrespondents), offer rewards and motivation for encouraging responses, train better the interviewers, scrutinise the questionnaires to be sure that the form has been filled correctly and have an accurate questionnaire construction.

Types of probability samples

3.1 Simple Random Sampling

Simple random sampling provides a natural starting point for a discussion of probability sampling methods, not because it is widely used, but because it is the simplest method and it underlies many of the more complex methods (Kalton 1983)[see 3]. The definition states that a simple random sampling is a subset of individuals chosen from a population. Each single person in this sample is chosen randomly and entirely by chance. Therefore, as a principle, they have the same probability of being chosen at any stage during the sampling process and vice versa. For example, suppose N elderly people want to get a ticket for a concert, but there are only X<N tickets for them, so they decide to have a fair way to decide who gets to go. Then, every elderly person gets a number in the range between 0 and N-1, and random numbers are generated, either electronically or from a table of random numbers. Thus, the first X numbers would identify the lucky ticket winners. This type of probability sample is commonly used without replacement in both small and large populations. Especially, for large samples this method can be used with replacement while obtaining same results because the probability of drawing the same person is very small. Advantages of this type are that is free of classification error, it requires minimum advance knowledge of the population other than the frame and it allows one to draw externally valid conclusions about the entire population. Nevertheless, the survey conductor should be careful to make an unbiased random selection of individuals so that if a large number of samples were drawn, the average sample would accurately represent the population. Generally, it is appropriate to use this method because its simplicity makes it relatively easy to interpret data collected in this manner and it best suits situations where not much information is available about the population and data collection can be efficiently conducted

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