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Roadtrip 5 by Searunner. sv -
Convenience sampling selection is based on the ease or "convenience" of gaining access to a sample. Rather than using a random or purposeful approach to sampling, the evaluator simply gathers data from people who are readily available. This form of data collection works for some areas of study, but using these options may result in low credible cases that produce inaccurate data and questionable results.
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Yes Or No Be Decisive photo by Michael Trolove
Confirming and disconfirming cases assist the evaluator in the confirmatory fieldwork stage of an evaluation. After gathering initial data, exploring the data and identifying patterns, the evaluator will start to develop findings. Confirming and disconfirming cases allow the evaluator to further refine and test initial findings. Both types of cases are important, confirming cases to provide deeper insights to preliminary findings and disconfirming cases to test and highlight the boundaries of the findings.
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Untitled by Pieterjan Grobler
Theory-based sampling involves selecting cases according to the extent to which they represent a particular theoretical construct. Purposive sampling is used as the population of the particular theoretical construct is difficult to determine.      
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Las bolas del Atomium photo by vcastelo on Flickr
Criterion sampling involves the identification of particular criterion of importance, articulation of these criterion, and systematic review and study of cases that meet the criterion.
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Large Snowball, London N14 photo by Christine Matthews
'Snowball or Chain Sampling is an option for locating information rich key informants and critical cases. The process begins by asking well situated people “Who knows a lot about ___?Whom should I talk to?” By asking a number of people who else to talk with, the snowball gets bigger and bigger as you accumulate new information-rich cases. 
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One Foot Island, Aitutaki, Cook Islands photo by g-hat on Flickr
A critical case is one that permits analytic generalisation, as, if a theory can work in the conditions of the critical case, it's likely to be able to work anywhere. Characteristics of particular cases may make them critical – level of education of the population, level of pollution of the environment, level of resistance to government intervention of a community. The purpose of the evaluation is to investigate the success of the program in this particular critical case. Commissioners of the evaluation may be interested in the results of the evaluation for logical generalisation to other sites.