The gamma distribution, of which the chi-squared and exponential distributions are particular cases, is used as a model in various statistical applications. Gupta (1960) has reviewed some of its applications to life tests, extreme values, reliability, and maintenance. Gupta & Groll (1961) have described the use of the gamma distribution in acceptance sampling based on life tests. It has also been used as an approximation to the distribution of quadratic forms in certain cases (see, for example, Box (1954) and references therein). A probability plotting procedure for the gamma distribution has been presented by Wilk, Gnanadesikan & Huyett (1962), and applications to life-test data analysis and to the statistical assessment of homogeneity of variance have been described. A method has been proposed by Wilk & Gnanadesikan (1961) for the graphical analysis of multi-response data which is based on the gamma distribution and involves the estimation of the shape parameter from order statistics. The present paper is concerned with the maximum-likelihood estimation of the scale and shape parameters of a gamma distribution, whose origin parameter is known (say equal to zero), based on order statistics. Tables are provided to facilitate obtaining these estimates and their use is illustrated and discussed. The case of unknown origin parameter is also briefly dealt with. Greenwood & Durand (1960) have considered the problem of maximum-likelihood estimation for the gamma distribution based on a complete sample and have presented tables for that case. Chapman (1956) considered the problem of maximum-likelihood estimation of the parameters of a truncated gamma distribution using a complete sample. The tables of the present paper give those of Greenwood & ]Durand (1960) as a special case and a particularly simple interpolation procedure is given for the circumstance of a complete sample. Two estimation situations involving order statistics may be distinguished: namely, when the size of the complete sample is known, and when the sample size is not known. In the sequel, the former case is considered in the context when the information available involves the 'smallest' observations only. The case where the size of the complete sample is unknown is still being investigated. The problem is formally stated in ? 2, and ?? 3 and 4 contain a discussion of the maximum likelihood estimation of the parameters and some attendant issues. ? 5 presents some special results for the case of estimation using the complete sample. Numerical approximations used are given in Appendix A. Tables to facilitate the solution of the maximumlikelihood equations are given in Appendix B. ? 6 presents some examples of the use of these tables. The case when the origin parameter is also unknown is briefly considered in ?7.
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