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### 25 Cards in this Set

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 probability sampling Based on the principle of randomness and probability theory Objective: to obtain a sample from a population that will provide useful information about the total population Objective: To obtain a representative sample Used when researchers want precise, statistical descriptions of large populations. A sample of individuals from a population must contain the same variations that exist in the population. types of probability sampling Simple random sampling (SRS) Systematic sampling Stratified sampling Multistage cluster sampling simple random sampling It is the most basic probability sampling method. It can be done by a computer. It needs an accurate sampling systematic sampling It is basically SRS with some modifications. The first step is to number all the elements in the sampling frame. Start out with a random number and then select every kth element Example: a sampling frame with 900, and you need 300 elements, the interval (or kth) will be 3 (900/300), so you select every 3rd element from the list Arrangement of elements in the list can result in a biased sample because of periodicity stratified sampling Grouping of units composing a population into homogenous groups before sampling. This procedure, which may be used in conjunction with simple random or systematic sampling, improves the representativeness of a sample in terms of the stratification variables. Rather than selecting sample for population at large, researcher draws from homogenous subsets of the population. Results in a greater degree of representativeness by decreasing the probable sampling error. The researcher ensures that appropriate numbers of elements are drawn from homogenous subsets of the population. Example: personnel in a school Multistage Cluster Used when it's not possible or practical to create a list of all the elements that compose the target population (sampling frame). Involves repetition of two basic steps: listing and sampling. The researcher draws several samples in stages (introducing error every time) Highly efficient but costly. Example: Immigrant survey in Non-Probability sampling Technique in which samples are selected in a way that is not suggested by probability theory. When there is no sampling frame available When studies have different objectives, such as how people understand certain things, meaning, etc.—closely aligned with qualitative methods. Examples include reliance on available subjects as well as purposive (judgmental), quota, and snowball sampling. types of non-prob sampling- Reliance on available subjects: Only justified if more accurate sampling methods are not possible. Researchers must exercise caution in generalizing from their data when this method is used. Example: Taking a poll in a street, using classmates in a class snowball sampling Appropriate when members of a population are difficult to locate. Researcher collects data on members of the target population she can locate, then asks them to help locate other members of that population. Example: study of stay Purposive or judgmental sampling Selecting a sample based on knowledge of a population, its elements, and the purpose of the study. Used when field researchers are interested in studying cases that don’t fit into regular patterns of attitudes and behaviors Example: study of immigrants Quota sampling Begin with a matrix of the population. Data are collected from people with the characteristics of a given cell in the matrix. Data should represent the total population. Example: hair study Illistrations looking at actual sampling design used to select a sample of university students. followi9ng steps and decisions involved in selecting that sample study population and sampling frame- illistrations Sampling frame: the actual list from which the elements will be selected. Examples: directories, registry lists, phone books If the sample is to be representative of the population, it is essential that the sampling frame include all (or nearly all) members of the population. Unit of observation: the element from which the observation is collected that provides the basis for analysis stratification- illistrations stratification by college class would be sufficient, although the students mikght have been further stratified within class, if desired, by gender, college, mahor... sample selection- illistrations once students have been arranbged by class, a systematic sample was selected across the entire rearranged list. once the sample has been selecdted, the computer was instructed to pring each student's name and mailing address sample modification before mailing questionares, researchers discovered that unexpeced expenses in the production of the questionaries made it impossible to cover the costs of mailing all 1100. so it was reduced sampling unit Element or set of elements considered for selection in some stage of parameter Summary description of a given variable in a population. Any characteristic of the population, such as percentage of people of different ethnicities in a city, the average income of a country. Importantly, the parameter is never known with accuracy for a large population; we must estimate it on the basis of samples stastic Summary description of a variable in a sample. A characteristic or characteristics (variables) by which we can describe a sample. They are used to estimate population parameters sampling error The degree of error of a given sample design. It is the deviation between sample results and population parameters due to random process. Sampling error size is affected by the size of the population and variation in the population. A large sample produces a smaller sampling error and a homogeneous population produces smaller a sampling error (Sampling error is often used to assess the quality of estimates: margin of error) population Study population: the larger pool or universe that he researcher is interested in. It is a theoretically specified aggregation or population Study population: the pool or an aggregation of elements from which the sample will be collected probability theory If many independent random samples are selected from a population, the sample statistics provided by those samples will be distributed around the population parameter in a known way (bell curve) If we were to select a large number of good samples, we would expect them to cluster around the true value (50%), but given enough such samples, a few would fall far from the mark. probability theory Researchers usually draw only one sample, but can use the principles of probability theory to estimate how close (or far) they have come to the actual population parameter because of the use of random sampling area under the curve 68% of the area lies within +_ 1 standard error from the population parameter 95% of the area lies within +_ 2 standard error from the population parameter 99% of the area lies within +_ 3 standard error from the population area under the curve 95% of samples will give estimates within the confidence interval of +_2 standard error I am 95% confident that my estimate will fall within the interval of +_ 2 standard errors Or a sample has a 95% chance of falling within +_2 standard errors (our confidence interval).