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Spaced retrieval, also known as expanded retrieval, is a learning technique, which requires users to rehearse information to be learned at different and increasing spaced intervals of time. In testing this type of learning, people are instructed to rehearse a given set of information at a certain time, and each new rehearsal is expected to have a longer period of time between itself and the previous rehearsal or an equal amount of time between rehearsals. At the end of every trial period there is a test phase. Landauer and Bjork first studied this technique of learning in 1978. The study required participants to learn names from flash cards. Prior to learning participants were placed into five different rehearsal types: uniform short, uniform moderate, uniform long, expanding, and contracting. These all indicate the amount and spacing of trials between each test. Uniform trials involve a number of trials between each test stage, but the trial numbers are fixed (e.g. 2-test-2-test-2-test). Contracting rehearsals involve larger intervals of time between the first few trials and the test phase, but eventually the trials decrease in number. Expanding involves starting with trials and tests close together, and as they progress the person would have more time between each trial and test (e.g. 1-test-2-test-3-test). The effectiveness of the rehearsal types was measured by seeing how accurately participants responded during a test phase. Expanding was proven to be the most important because it produced the highest amount of recall during the test period. The data behind this initial research indicated that an increasing space between rehearsals (expanding) would yield a greater percentage of accuracy at test points. Spaced retrieval with expanding intervals is believed to be so effective because with each expanded interval of retrieval it becomes more difficult to retrieve the information because of the time elapsed between test periods; this creates a deeper level of processing of the learned info in long term memory at each point. Another reason that the expanding retrieval model is believed to work so effectively is because the first test happens early on in the rehearsal process. The purpose of this is to increase retrieval success. By having a first test that followed initial learning with a successful retrieval, people are more likely to remember this successful retrieval on following tests. Although expanding retrieval is commonly associated with spaced retrieval, a uniform retrieval schedule is also a form of spaced retrieval procedure. Spaced retrieval is typically studied through the use of memorizing facts. Traditionally speaking, it has not been applied to fields that required some manipulation or thought beyond simple factual/semantic information. A more recent study has shown that spaced retrieval can benefit tasks such as solving math problems. In a study conducted by Pashler, Rohrer, Cepeda, and Carpenter, participants had to learn a simple math principle in either a spaced or massed retrieval schedule. The participants given the spaced retrieval learning tasks showed higher scores on a final test distributed after their final practice session. This is unique in the sense that it shows spaced retrieval can be used to not only remember simple facts or contextual data, but it can also be used in fields, such as math, where manipulation and the use of particular principles or formulas (e.g. y = mx + b) is necessary. These researchers also found that it is beneficial for feedback to be applied when administering the tests. When a participant gave a wrong response, they were likely to get it correct on following tests if the researcher gave them the correct answer after a delayed period. Spaced retrieval is a useful tool for learning that is relevant to many domains such as fact learning or mathematics, and many different tasks (expanding or uniform retrieval). Many studies over the years have contributed to the use and implementation of spaced retrieval, and it still remains a subject of interest for many researchers.