variance

  1. C

    Assumptions for comparing means and which test to use

    I am attempting to compare the means of soil moisture measured from two adjacent soil profiles at both 10cm and 30cm. Soil measurements were measured discretely six times over the course of two months at the same date and time for each profile. However, when looking to perform tests to compare...
  2. X

    Expected Value + Variance

    Hi guys just a quick question I am told i have two values which are 100 with probability of 0.8 and 1000 with probability of 0.2. Now, I know the E(X) is just 100(.8) + 1000 (.2). I know my var(x) = e(x^2) - e(x)^2 so is my var(x) just 100^2(.8) and 1000^(.2) ?
  3. I

    Normalizing data before calculating variation?

    Hello, We want to identify for which categories we need to differentiate in assortment over stores. We want to prioritize categories from highest potential to lowest (since we deal with 200 categories we can not do all categories at once). One of the things we want to take into account is...
  4. R

    standard deviation greater than variance, is this ok?

    I am putting together some audit results and it was suggested I present them as mean +/- SD. However there is a significant variance in the results. For example looking at hours it took to achieve a targeted temperature goal : results varied from 0 hours to 11. Using excel I ended up though with...
  5. A

    Help with unbiased estimators

    I'm working on a homework problem and the question I'm stuck on is as follows: "Give an unbiased estimator for 2µ+3σ^2" I'm not sure how I would go about doing that since the mean and the variance make up the function.
  6. C

    Variance / Anova Questions

    I have readings taken at 20 different positions for 3 different treatments ( x 3 reps). The mean for each treatment is different. I am not interested in testing for significant differences in the means, i am interested only in the level of variation. a) I want to determine if the level of...
  7. M

    ARCH-process with exponencial variance function

    I need some help for a proof regarding Engles ARCH-paper from 1982. For an ARCH-process with a variance function h_t = exp(a_0 + a_1 y_{t-1}^2) he states, that the data generatet from this model has infinite variance (or goes to infinity) whenever a_1 is not zero. I need to explain why...
  8. A

    reliability effects components of variance

    Hello TalkStats community. I'm currently stuck with a problem, concerning how reliability of a process effects the variance of the process. let me give you an example: Lets assume there is a machine that produces a product X. the machine and runs for 1000 hours. However, the machine has...
  9. V

    Find costant that satisfies the variance

    Hi, I have this regression model : Y_i=B_1+B_2logx_i+2e_i e_i~N(0,σ2) I am asked to find the constant : Y∗=y c that satisfies V(Y∗)=σ^2 I have some doubts ... I would say that the constant is equal to 1/2 ?
  10. A

    Working out standard deviation for three random variables with different sized ranges

    X = {1,2,3,4,5,6}, Y = {1,2,3,4}, Z = {2,4,6,8}. The question is: work out standard deviation of A = 2S-1, where S = the sum of the three random variables. I know that E(X) = 3.5, E(Y) = 2.5, E(Z) = 5 Var(X) = 2.916, Var(Y) 1.25, Var(Z) = 5 Now in order to find out the standard...
  11. D

    Minimum P to sample boxes

    I have N boxes, each of which may or may not contain a ball. I want to know how many boxes have balls by sampling the boxes in the following way. For each box in the N boxes: I open this box with probability P If at the end I find out K boxes have a ball, I estimate the number of boxes...
  12. G

    Systematic component variation

    The appendix of the paper of [McPherson et al (1982) contains a derivation of the systematic component variation SCV. I understand the derivation with exception of the first step. Here are the premisses: O_i: observed cases in region i E_i: expected cases in region i \lambda_i...
  13. S

    Variance main effect influencing interaction effect

    Does variance of one of the two main effects in a 2-way (ANOVA) interaction influence the significance of the interaction effect? Two analyses are being compared: of brain area A and brain area B. For both brain areas A & B, a 2-way AVOVA is being performed: Treatment (2) x Valence (3)...
  14. F

    Estimate individual variance from pool variance

    Let's say I have a group of 100 tiny beads, and I want to know the variance in their weight. However, my scale can't weigh one bead at a time, so I weight them in non-overlapping pools of 10 beads each. Then I can calculate the variance among pools, but what I really want to know is the...
  15. U

    Calculating the bias of variance estimator

    Dear all, I have been given two variance (S^2) estimators, S1 and S2. I am not including the square (^2) as it may cause confusion. And I am to determine the bias of these estimators. Now, given that estimator S1 has the same equation as sample variance, it should therefore be classed as...
  16. S

    impose internal coherence (positive definite) on a matrix

    how do I turn a matrix which bears the resemblance of a variance covariance matrix into a "positive definite" matrix (=a usable var cov matrix) ? is there any way?
  17. C

    Are two sources of variation ever completely independent?

    Pick three values to represent the true value, and two error sources and put them as the header in separate columns. Remember to include a positive or negative sign to represent the direction of the error. Let the experimentally determined value be the sum of all three values. Now, use a...
  18. M

    Violating Homogeneity

    Hi, if in regression models homogeneity is violated, the variance can in principle depend on both: Predictor and outcome variable(s). In case of a dependency on categorical or continuous predictor variables, there are many ways to model different dependencies, e.g. using GLS models. In...
  19. K

    Probability of a Poisson Distribution without t given problem

    This is on my final exam study guide for statistics. I have absolutely no idea how to do it Problem 1: Valerie Variance is taking her "no-time limit " final exam. As always, the exam has an infinite number of questions and the instructor gives no partial credit. Valerie being a diligent...
  20. S

    Comparing absolute values of difference scores between two groups

    I have two groups (A and B) that each completed a different version of an experimental measure (measure A and measure B). Both groups also completed a standard, "gold standard", measure. For the purposes of my study, I am treating the "gold standard" measure as a participant's true score. If I...