Sample Means
How do I take samples of random variables?
- It's easier to explain this with an example
- Let where is the number of heads in 10 flips of a fair coin
- heads
- Imagine doing this experiment each day for 7 days
- is the number of heads on day 1
- is the number of heads on day 2
- ...
- is the number of heads on day 7
- to are independent random variables, each with identical distributions
- Identical distributions don't make the number of heads on each day identical
- is called the population distribution
- With population mean
- And population variance
- is called the random sample of size 7 taken from the population distribution
- Where each is an independent identical distribution
What is the sample mean?
- Let , be a random sample of size taken from the population distribution
- The sample mean is given by
- It is not a fixed number
- Different samples of size have different sample means
- From the example before
- Each day there's a different number of heads
- Here's one sample: 4, 6, 5, 5, 3, 5, 6
- This particular sample mean is 4.857...
- This is close to the population mean of heads
- A second sample of 7 days would give a different sample mean
- As would a third sample of 7 days
- Generating lots of samples like this will give a distribution of sample means
- Each day there's a different number of heads
What is the distribution of the sample mean?
- If a random sample of size , , is taken from a normal population distribution,
- Then the distribution of the sample mean is
- Where
- And are independent
- The mean of the sample mean distribution is the same as the population mean
- The variance of the sample mean distribution is the population variance divided by
- So larger samples are better, as their sample means are closer to (less spread out)
- Then the distribution of the sample mean is
- This only holds when the population is normal