Abstract
Dependence between the variables can seriously affect the confidence level when the traditional t-statistic is used to cornstruct confidence limits for the expectation of the variables. The objective of the present paper is to Investigate the normal approximations fo possible modifications of the t-statistic with respect to their use for the construction of confidence intervals. The methods consist in the replacement of the usual variance estimator by a linear combination of estimated covariances, though with different weights and with different rationales. The methods are shown to give morc correct confidence levels than the unmodified t-statistic. Comparing the methods. merits for either one are found. The choice is to some extent dependent upon the underlying model. Some indications about the sample size necessary for the approximation to work at certain levels of satisfaction are given.

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