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| consists of methods for organizing and summarizing information |
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| The collection of all individuals or items under consideration in a statist ical study |
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| That part of the population from which information is obtained |
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| consists of methods for drawing and measuring the reliability of conclusions about a population based on information obtained for a sample of the population |
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| obtain information on the entire population of interest |
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| two methods used in finding a census |
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| sampling and experimentation |
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| should reflect as closely as possible the relevant characteristics of the population under consideration |
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| a random device, such as tossing a coin or consulting a table of random numbers, is used to decide which members of the population will constitute the sample instead of leaving such decision to human judgement |
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| A sampling procedure for which each possible sample of a given size is equally likely to be the one obtained |
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| A sample obtained by simple random sampling |
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| sampling with or without replacement |
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| assume that random sampling is done without replacement |
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| A table of randomly chosen digits |
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| Systematic Random Sampling |
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Definition
1) Divide the population size by the sample size and round the result down to the nearest whole number, m.
2) Use a random-number table (or a similar device) to obtain a number, k, between 1 and m.
3) Select for the sample those member of the population that are numbered k, k + m, k + 2m,...
2) Use a random |
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1) Divide the population into groups (clusters)
2) Obtain a simple random sample of the clusters
3) Use all members of the clusters obtained in step 2 |
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more reliable than cluster.
Population is first divided into subpopulations, called strata, and then sampling is done from each stratum |
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| In stratified sampling, the strata are often sampled in proportion to their size. |
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| Stratified Random Sampling With Proportional Allocation |
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The simplest type of stratified sampling
1) Divide the population into subpopulations (strata)
2) From each stratum, obtain a simple random sample of size proportional to the size of the stratum; that is, the sample size for a stratum equals the total sample size times the stratum size divided by the population size.
3) Use all numbers obtained in Step 2 as the sample |
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| Researchers simply observe characteristics and take measures, as in a sample survey |
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| Researchers impose treatments and controlsand then observe characteristics and take measurements. |
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| Researchers impose treatments and controlsand then observe characteristics and take measurements. |
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| Difference between observation study and designed experiments |
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| Observational studies can reveal only association, whereas designed experiments can help establish causion |
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| Principles of Experimental Design |
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| Control, Randomization, and Replication |
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| Some method should be used to control for effects due to factors other than the ones of primary interest |
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| Subjects should be randomly divided into groups to avoid unintentional selection bias in consulting the groups, that is, to make the groups as similar as possible |
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| A sufficient number of subjects should be used to ensure that randomization creates groups that resemble each other closely and to increase the chances of decting differences among the treatments when such differences actually exist. |
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| The group receiving the treatment |
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| group receiving the placebo |
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| Experimental Units; Subjects |
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| In a designed experiment, the individuals or items on which the experiment is performed are called experimental units. When the experimental units are human, the term subject is often used in place of experimental unit. |
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| The characteristic of the experimental outcome that is to be measured or observed |
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| A variable whose effect on the response variable is of interest in the experiment |
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| the possible values of a factor |
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| Each experimental condition. For one-factor experiments, the treatments are the levels of the single factor. For multifactor experiments, each treatment is a combination of levels of the factors |
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| Completely Randomized Design |
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| In a completely randomized design, alln the experimental units are assigned randomly among all the treatments. |
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| In a randomized block design, the experimental units are assigned randomly among all the treatments separately with in each block |
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