Types of Samples Dr. Sa’ed H. Zyoud

Determining Sample size A Sample is a segment of the population selected to represent the population as a whole. the sample should be -representative and -allow the researcher to make accurate estimates of the thoughts and behaviour of the larger population

Determining Sample size Designing the sample calls for three decisions: Who will be surveyed? ( The Sample) The researcher must determine what type of information is needed and who is most likely to have it. How many people will be surveyed? (Sample Size) Large samples give more reliable results than small samples. However it is not necessary to sample the entire target population.

Determining Sample size How should the sample be chosen? (Sampling) Sample members may be chosen at random from the entire population (probability sample) The researcher might select people who are easier to obtain information from (nonprobability sample)

Types of Samples Probability samples Simple random sample: Every member of the population has a known and equal chance of being selected. Table of random numbers (e.g. x-ray film) Computer generated list of random number

where n is the sample size, and N is the population size. Probability samples Systematic random sample The most common form of systematic sampling is an equal-probability method. In this approach, progression through the list is treated circularly, with a return to the top once the end of the list is passed. The sampling starts by selecting an element from the list at random and then every kth element in the frame is selected, where k, the sampling interval (sometimes known as the skip): this is calculated as: k= N/n where n is the sample size, and N is the population size.

Probability samples Stratified random sample :Population is divided into relevant strata (subgroups) such as age groups and random samples are drawn from each group. Cluster (area) sample: The population is divided into clusters (blocks) and subset of clusters is randomly selected, and the researcher draws a sample of the group to interview (e.g. geographic areas or districts)

Types of Samples Nonprobability samples Convenience sample: The researcher selects the easiest population members from which to obtain information. Judgment sample: The researcher uses his judgment to select population members who are good prospects for accurate information. Quota sample (as strata): The researcher finds and interviews a prescribed number of people in each of several categories.