Is the sample 30 there are several different

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There are several different scenarios which the question of which confidence interval should be used. "One interpretation is to always use Z when the sample size is greater than or equal to 30 and t when it is less than 30. A contradictory rule is to use the t distribution wheneverthe population standard deviation is unknown and Z when it is known, regardless of the sample size." (Ozgur and Strasser, 2004, p. 178-179). Continually referred to as Rule 1(sample size) andRule 2 (if σ of the population is known). It seems that text books, seem to push the hard-fast rule
based on population size question. "In the rare case that the population standard deviation (σ) is known, there is universal agreement that Z is the correct statistical test as long as the sample is obtained from a normally distributed population." (Ozgur and Strasser, 2004, p. 178). If that was the case, since the populations standard deviation is rarely know, it would be logical that we would rarely use the Z interval, but that is not the case, so there must be another method of determining which one to use. Ozgur and Strasser have come to the conclusion that over the past

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