Edexcel A Level Maths: Statistics:复习笔记5.3.1 Sample Mean Distribution

Sample Mean Distribution

What is the distribution of the sample means?

  • For any given population it can often be difficult or impractical to find the true value of the population mean, µ
    • The population could be too large to collect data using a census or
    • Collecting the data could compromise the individual data values and therefore taking a census could destroy the population
    • Instead, the population mean can be estimated by taking the mean from a sample from within the population
  • If a sample of size n  is taken from a population, X, and the mean of the sample, is calculated then the distribution of the sample means,  , is the distribution of all values that the sample mean could take
  • If the population, X,  has a normal distribution with mean, µ , and variance,   , then the mean expected value of the distribution of the sample means,  would still be µ but the variance would be reduced
    • Taking a mean of a sample will reduce the effect of any extreme values
    • The greater the sample size, the less varied the distribution of the sample means would be
  • The distribution of the means of the samples of size taken from the population, will have a normal distribution with:
    • Mean,
    • Variance
    • Standard deviation
  • For a random variable the distribution of the sample mean would be
  • The standard deviation of the distribution of the sample means depends on the sample size, n
    • It is inversely proportional to the square root of the sample size
    • This means that the greater the sample size, the smaller the value of the standard deviation and the narrower the distribution of the sample means

Worked Example

A random sample of 10 observations is taken from the population of the random variable and the sample mean is calculated as .  Write down the distribution of the sample mean,  .

Exam Tip

  • Look carefully at the distribution given to determine whether the variance or the standard deviation has been given.

 

 

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