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Prior-year income is used because it is a good predictor of current-year income and is verifiable . Focus groups were used because they are able to elicit more precise and indepth information about the experiences and opinions of hard-to-reach populations than surveys . Liquid nitrogen is used because it is relatively inexpensive , but it also requires no refrigeration , thus cutting down on energy needed . ES cells are used because they are able to differentiate into nearly any type of adult cell , which means that if a gene is knocked out in an ES cell , the effects can be observed in any tissue in an adult mouse . " Therefore , an is used because it is acting as an article proceeding a word beginning with a vowel sound .