standard-deviation

问题 When generating random numbers in R using rnorm (or runif etc.), they seldom have the exact mean and SD as the distribution they are sampled from. Is there any simple one-or-two-liner that does this for me? As a preliminary solution, I've created this function but it seems like something that should be native to R or some package. # Draw sample from normal distribution with guaranteed fixed mean and sd rnorm_fixed = function(n, mu=0, sigma=1) { x = rnorm(n) # from standard normal distribution

问题 Does LINQ model the aggregate SQL function STDDEV() (standard deviation)? If not, what is the simplest / best-practices way to calculate it? Example: SELECT test_id, AVERAGE(result) avg, STDDEV(result) std FROM tests GROUP BY test_id 回答1: You can make your own extension calculating it public static class Extensions { public static double StdDev(this IEnumerable<double> values) { double ret = 0; int count = values.Count(); if (count > 1) { //Compute the Average double avg = values.Average(); /

问题 A simple example of calculating standard dev: d <- c(2,4,4,4,5,5,7,9) sd(d) yields [1] 2.13809 but when done by hand, the answer is 2. What am I missing here? 回答1: Try this R> sd(c(2,4,4,4,5,5,7,9)) * sqrt(7/8) [1] 2 R> and see the rest of the Wikipedia article for the discussion about estimation of standard deviations. Using the formula employed 'by hand' leads to a biased estimate, hence the correction of sqrt((N-1)/N). Here is a key quote: The term standard deviation of the sample is used

问题 For each X -value I calculated the average Y -value and the standard deviation ( sd ) of each Y-value x = 1:5 y = c(1.1, 1.5, 2.9, 3.8, 5.2) sd = c(0.1, 0.3, 0.2, 0.2, 0.4) plot (x, y) How can I use the standard deviation to add error bars to each datapoint of my plot? 回答1: A Problem with csgillespie solution appears, when You have an logarithmic X axis. The you will have a different length of the small bars on the right an the left side (the epsilon follows the x-values). You should better