standard deviation

  1. D

    Standard Error - interpretation and relative standard error

    Hi, I am unclear about how to interpret the Standard Error of the Mean (SEM) directly. For example, when a mean is reported as 5.00 + 0.50SEM, how do you directly relate the 0.50 to 5.00? To help address this question I have written dot points about what I know about the SEM and interspersed...
  2. L

    Determining the Standard Deviation (vice Standard Error) of regression coefficients?

    This is a question I’ve been wondering about for a while, but perhaps it’s not possible or perhaps I just need a point in the right direction. I constructed the attached mock data samples with four variables, A, B, C, D, each of which is either present (1) or not (0). The true value for each...
  3. R

    Calculation of Levene's Robust Test of Equality of Variances (not in SPSS!)

    Hi. I'm trying to understand the calculation of a Levene's test of homogeneity of variance, for a section of my thesis on sex differences in variability of cognitive performance (greater male variability hypothesis). As I don't have the original data I'm analysing, I can't use SPSS to...
  4. A

    Test whether the SD of multiple normal distributions differs between two genes

    Hi, I am comparing data on gene expression between two different genes that have been tagged with GFP. Gene expression is measured using flow cytometry so that gene expression is a continuous variable along the x-axis, with counts on the y-axis. For each gene I thus have a normal distribution...
  5. S

    New to statistics, need a few steps on solving problem

    Okay guys, as the title says, I'm pretty new to stats. Currently I am working on a problem for a college class that is giving me a lot of grief. I would greatly appreciate some input. Here it is: A sample of 65 male professors at a major four-year university produced a mean annual pay of...
  6. L

    Help with probability, sample mean & standard deviation

    Can anyone help me on how to do this problem? I'm so confused
  7. V

    Standard deviation of Sample vs. Standard deviation of mean

    Hi, I have a very basic confusion. What exactly is the difference between them? Can you also give a simple example? Thanks guys
  8. P

    Error on the standard deviation, skewness & Kurtosis

    Dear all, Given a sample of N points extracted from a distribution, I would like to know how to estimate the error on the higher moments of the distribution, such as the standard deviation, the skewness and kurtosis. In other words, I know the error on the mean is simply sigma/sqrt(n), but...
  9. S

    Binomial Distribution

    I am in dire need of some help with binomial distribution! In a given year, the probability that an adult American male dies in a car accident is equal to 0.0002. a) In a city having 1 million adult American males, state assumptions for a binomial distribution to apply to X= the number of them...
  10. P

    Combining standard deviation/normal distribution (do I have this right?)

    I have a variable with a mean of 52% and a standard deviation of 2.5%. However, the 52% mean was derived from an opinion poll which itself has a 3% margin of error, which I take to imply a standard deviation of 1.53%. Assuming a normal distribution I can calculate from the former a 21% chance of...
  11. C

    Standard deviations related question...help?

    Q. A school social worker is determining eligibility for anxiety management groups at a high school she is employed at. If she is interested in identifying the percentage of potential clients who scored higher than 1.5 standard deviations above the mean on a standardized anxiety index, for the...
  12. O

    Calculating probability given mean and standard deviation

    Hi I have looked around for this post on the forum but could not find one and have spent a while attempting this question: Given that X is normally distributed with mean 100 and standard deviation 9 and given a sample size of 16, compute the following: 1) mean and variance of X-dash 2)...
  13. H

    To combine or not to combine standard deviations?

    Hi, I'm testing the effect of 115 Chinese herbs on bacterial biofilms. I have been carrying out my experiment in microtiter plates and measuring the absorbance readings. My test plate consists of bacterial culture + herb (six repeats of each herb) While my background reading plate...
  14. X

    Difference between sample and population

    Now, in this case, will the given data for each group considered as sample or population ? I want to calculate SD, MD and Variance and i am getting confuse shall i consider it as a population or sample.
  15. victorxstc

    Contributions to writing code that enable R to analyze the "Descriptive" statistics

    Apparently R does use only the raw data. In some other statistical programs, there is the option to calculate some simple tests, only by inputting the descriptive statistics. I for one think it would be much better if we could enable R to run such tests too. There are not many tests that can...
  16. M

    Is it allowed to take the standard deviation of percents?

    Hello, I've a data set of 1002 (I'm quite sure it is not a normal distribution). Foreach entry i've some Value in %. I've calculated the mean of the percent values. Can I calculate the standard deviation of this percent values? Do I need a different formula then the normal one...
  17. T

    Normal distribution, Z-Score and Probability Help.

    I'm having a difficult time trying to answer these questions. Any help would be very helpful. The time it takes a randomly selected job applicant to perform a certain task has a normal distribution with mean 120 seconds and standard deviation 12 seconds. 1. We are interested in computing...
  18. C

    Citation needed for removing 3 Stdev outliers

    Hi, In a research paper, I had remove values that were 3 standard deviations away from the mean. The values simply didn't make any sense. I have been searching all over for a citation supporting the 3 standard deviation rule but haven't found much as most researchers don't seem to include...
  19. G

    Standard deviation - how many samples to I need to measure to get a "good" one?

    I was looking at Minitab 16's Power and Sample Size calculation tool for a one sample t-test. If you're not familiar with it, you can put in 2 of the 3 factors: sample size, differences, and power values. You also have to put in the standard deviation. This got me thinking. How do you know...
  20. J

    Standard deviation as a percentage

    Hi i'm new here and have a really quick question, How do you interpret a standard deviation given as a percentage (in this case 20%)? I need to use the variance in further calculations (working out type i and ii errors), the data is normally distributed, and the sample size is 30, i have...