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Homework Help: Determining a confidence interval for data.

  1. Oct 18, 2012 #1
    1. The problem statement, all variables and given/known data
    I have 20 data points of the surface roughness of oil pipes, the mean, and the std of the sample.

    I need to get a 95% confidence interval of the mean surface roughness of these pipes. I did a boxplot, and found that it's negatively skewed, and therefore not normally distributed:


    Would using the t-table be OK? I'm usually used to doing this for data when it is normally distributed, I'm not too sure what to do when it's not normal.

    This is what I did:

    x ± t-val(s/√n)

    Where the t-val is t19 and I looked up the t-table for the value. s = std and n = 20 and x is of course the mean.
  2. jcsd
  3. Oct 22, 2012 #2
    when the sample size is greater than 30 you can use the t-confidence interval even if the population is not normal.
  4. Oct 22, 2012 #3
    several of my professors have said n=30 is a number that's been around for a while and is used as a rule of thumb, but you don't have to follow it super strictly. How skewed is the data? If it's not horribly skewed, if it's even vaguely bell-shaped, I'd imagine n=20 is acceptable. But please correct me if I'm wrong, I'm still new at this.
  5. Oct 22, 2012 #4
    I suppose n=20 is reasonable for using t-intervals
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