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 Oct 4 revised Least Squares Derivation Elaborated and gave a proof. Oct 4 answered Least Squares Derivation Oct 4 comment Why does negative binomial random variable uses ${n-1 \choose r-1}$ instead of ${n \choose r}$ as coefficient? No we are not skipping the last trial.However note that last trial is not guaranteed success,it is success with probability $p$.Hence we multiplied by $p$,otherwise we'ld have got ${n-1 \choose r-1} {(1-p)}^{(n-r)}p^{(r-1)}$ Oct 4 awarded Teacher Oct 4 comment Why does negative binomial random variable uses ${n-1 \choose r-1}$ instead of ${n \choose r}$ as coefficient? Consider again,if we use $n \choose r$ then it includes the possibility that first r were successes and remaining were failures,then we don't get nth trial as success.For nth trial to be rth success as we want,we only have freedom in choosing $r-1$ successes from $n-1$ trials as rth success is to occur at nth trial. Oct 4 answered Why does negative binomial random variable uses ${n-1 \choose r-1}$ instead of ${n \choose r}$ as coefficient? Oct 4 answered Finding the slope of a function Oct 4 answered how to find the quartiles Oct 4 awarded Editor Oct 4 revised Power series of a function answer was null and void. Oct 4 answered Power series of a function