# 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.