Cross Validated Weekly Newsletter - Monday, September 29, 2014

Top new questions this week: Explanation for non-integer degrees of freedom in t test with unequal variances The SPSS t-Test procedure reports 2 analyses when comparing 2 independent means, one analysis with equal variances assumed and one with equal variances not assumed. The degrees of freedom (df) when … hypothesis-testing t-test heteroscedasticity degrees-of-freedom t-distribution   asked by Joel W. 9 votes answered by gung 8 votes Is the sum of a number of ordinal variables still ordinal? I have performed a survey where I have a number of questions which can be answered Strongly Agree, Agree etc. to Strongly Disagree. Some of the questions have been designed to measure the same thing. … regression survey ordinal likert   asked by SamPassmore 7 votes answered by Glen_b 9 votes Why $Y$ should be transformed before the predictors? Both answers in these threads, one and two claim that $Y$ should be transformed before applying any other transformation to the predictors. Indeed Weisberg chapter on transformations focus more on DV … regression data-transformation   asked by Robert Kubrick 7 votes answered by Glen_b 5 votes Why is the standard deviation defined as sqrt of the variance and not as the sqrt of sum of squares over N? Today I taught an introductory class of statistics and a student came up to me with a question, which I rephrase here as: "Why is the standard deviation defined as sqrt of variance and not as the sqrt … variance standard-deviation basic-concepts teaching   asked by tomka 7 votes answered by whuber 7 votes Why does it take R a long time to fit a model with a many-level factor? I fit a model with a factor with many levels and it takes R a really long time to fit that model. Why is this? For example, if I fit a regression to predict players' salaries, and include a factor … regression categorical-data   asked by goldisfine 7 votes answered by Glen_b 8 votes Can the power of a test of an inconsistent estimator still go to 1 as N goes to infinity? I'm reading a comment to a paper, and the author states that sometimes, even though the estimators (found by ML or maximum quasilikelihood) may not be consistent, the power of a likelihood ratio or … mathematical-statistics references inference power consistency   asked by An old man in the sea. 6 votes answered by Glen_b 4 votes Proving the LATE Theorem of Angrist and Imbens 1994 Assume we have a binary instrument $Z_i$ which can be used to estimate the effect of the endogenous variable $D_i$ on the outcome $Y_i$. Suppose the instrument has a significant first stage, it is … mathematical-statistics econometrics proof instrumental-variables   asked by user44903 6 votes answered by Andy 4 votes Greatest hits from previous weeks: How to deal with omitted dummy variables in a fixed effect model? I am using a fixed effect model for my panel data (9 years, 1000+ obs), since my Hausman test indicates a value $(Pr>\chi^2)<0.05$. When I add dummy variables for industries that my firms … stata panel-data fixed-effects-model hausman   asked by BEF 7 votes answered by mpiktas 10 votes Making sense of principal component analysis, eigenvectors & eigenvalues In today's pattern recognition class my professor talked about PCA, eigenvectors & eigenvalues. I got the mathematics of it. If I'm asked to find eigenvalues etc. I'll do it correctly like a … pca intuition   asked by claws 188 votes answered by JD Long 133 votes Can you answer these? Computing a Metropolis-Hastings target distribution? In implementations of the Metropolis-Hastings algorithm, how is the target distribution $\pi(\mathbf{x}) = P(\mathbf{x}|\mathbf{e})$ computed or estimated while computing the acceptance probability … mcmc metropolis-hastings   asked by Eric Huang 2 votes Estimating number of balls by successively selecting a ball and marking it Lets say I have N balls in a bag. On my first draw, I mark the ball and replace it in the bag. On my second draw, if I pick up a marked ball I return it to the bag. If, however I pick up a … probability estimation population combinatorics capture-mark-recapture   asked by ashemon 6 votes Find the median individual with observations on multiple variables over time I have a question regarding the use of median. We collected data concerning disease development on hosts. We gathered the evolution of three variables over time on individuals. Their characteristics: … time-series multivariate-analysis median   asked by Thomas 3 votes

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