Which of the following statements are correct when deciding whether to use the z or the t distribution? I. Use t when the sample size is below 30 and population variance is unknown. II. Use z when we have a normal population and we know the standard deviation. IIII. Use z when population variance is unknown even when is n<30. IV. Use z when n<30 and population variance is unknown. * I, II, III and IV O II, III and IV O ,IlI and IV O , Il and I

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