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A IRS auditor randomly selects 3 tax returns from 45 of which 15 contains errors What is the probability that she selects none of those containing errors?
.2861
What is four sigma?
It could refer to four standard errors. If an observation from a Gaussian (normal) distribution is 4 standard errors away from the mean, it has an extremely low probability.
To cut the maximum likely error in half the sample size should be?
... should be increased by a factor of 4. Note that this implies that the only errors are statistical (random) in nature; increasing the sample size won't improve systematic errors.
Whats the difference between random errors and systematic errors?
Random errors - Random errors can be evaluated through statistical analysis and can be reduced by averaging over a large number of observations. Systematic errors - Systematic errors are difficult to detect and cannot be analyzed statistically, because all of the data is off in the same direction (either to high or too low). Spotting and correcting for systematic error takes a lot of care.
What is gaussian distribution and what is its significance in least squares analysis?
From a technical perspective, alternative characterizations are possible, for example: The normal distribution is the only absolutely continuous distribution all of whose cumulants beyond the first two (i.e. other than the mean and variance) are zero. For a given mean and variance, the corresponding normal distribution is the continuous distribution with the maximum entropy. In order to make statistical tests on the results it is necessary to make assumptions about the nature of the experimental errors. A common (but not necessary) assumption is that the errors belong to a Normal distribution. The central limit theorem supports the idea that this is a good approximation in many cases. The Gauss-Markov theorem. In a linear model in which the errors have expectation zero conditional on the independent variables, are uncorrelated and have equal variances, the best linear unbiased estimator of any linear combination of the observations, is its least-squares estimator. "Best" means that the least squares estimators of the parameters have minimum variance. The assumption of equal variance is valid when the errors all belong to the same distribution. In a linear model, if the errors belong to a Normal distribution the least squares estimators are also the maximum likelihood estimators. However, if the errors are not normally distributed, a central limit theorem often nonetheless implies that the parameter estimates will be approximately normally distributed so long as the sample is reasonably large. For this reason, given the important property that the error mean is independent of the independent variables, the distribution of the error term is not an important issue in regression analysis. Specifically, it is not typically important whether the error term follows a normal distribution. In a least squares calculation with unit weights, or in linear regression, the variance on the jth parameter, denoted , is usually estimated with where the true residual variance σ2 is replaced by an estimate based on the minimised value of the sum of squares objective function S. The denominator, n-m, is the statistical degrees of freedom; see effective degrees of freedom for generalizations. Confidence limits can be found if the probability distribution of the parameters is known, or an asymptotic approximation is made, or assumed. Likewise statistical tests on the residuals can be made if the probability distribution of the residuals is known or assumed. The probability distribution of any linear combination of the dependent variables can be derived if the probability distribution of experimental errors is known or assumed. Inference is particularly straightforward if the errors are assumed to follow a normal distribution, which implies that the parameter estimates and residuals will also be normally distributed conditional on the values of the independent variables.
