STA 291 Spring 2010 Lecture 12 Dustin Lueker

Central Limit Theorem For random sampling, as the sample size n grows, the sampling distribution of the sample mean, , approaches a normal distribution Amazing: This is the case even if the population distribution is discrete or highly skewed Central Limit Theorem can be proved mathematically Usually, the sampling distribution of is approximately normal for n≥30 We know the parameters of the sampling distribution STA 291 Spring 2010 Lecture 12

Central Limit Theorem (Binomial Version) For random sampling, as the sample size n grows, the sampling distribution of the sample proportion, , approaches a normal distribution Usually, the sampling distribution of is approximately normal for np≥5, nq≥5 We know the parameters of the sampling distribution STA 291 Spring 2010 Lecture 12

Example Assume the population mean is 15 with a standard deviation of 7. What is the probability of getting a sample mean greater than 13.5 from a sample of size 36? STA 291 Spring 2010 Lecture 12

Example Assume the true proportion of UK students who will fill out an NCAA tournament bracket is .62. What is the probability of finding 15 people out of a sample of 20 that are going to fill out an NCAA tournament bracket? STA 291 Spring 2010 Lecture 12

Statistical Inference: Estimation Inferential statistical methods provide predictions about characteristics of a population, based on information in a sample from that population Quantitative variables Usually estimate the population mean Mean household income For qualitative variables Usually estimate population proportions Proportion of people voting for candidate A STA 291 Spring 2010 Lecture 12

Two Types of Estimators Point Estimate A single number that is the best guess for the parameter Sample mean is usually a good guess for the population mean Interval Estimate Point estimator with error bound A range of numbers around the point estimate Gives an idea about the precision of the estimator The proportion of people voting for A is between 67% and 73% STA 291 Spring 2010 Lecture 12

Point Estimator A point estimator of a parameter is a sample statistic that predicts the value of that parameter A good estimator is Unbiased Centered around the true parameter Consistent Gets closer to the true parameter as the sample size gets larger Efficient Has a standard error that is as small as possible (made use of all available information) STA 291 Spring 2010 Lecture 12

Unbiased An estimator is unbiased if its sampling distribution is centered around the true parameter For example, we know that the mean of the sampling distribution of equals μ, which is the true population mean Thus, is an unbiased estimator of μ Note: For any particular sample, the sample mean may be smaller or greater than the population mean Unbiased means that there is no systematic underestimation or overestimation STA 291 Spring 2010 Lecture 12

Biased A biased estimator systematically underestimates or overestimates the population parameter In the definition of sample variance and sample standard deviation uses n-1 instead of n, because this makes the estimator unbiased With n in the denominator, it would systematically underestimate the variance STA 291 Spring 2010 Lecture 12

Efficient An estimator is efficient if its standard error is small compared to other estimators Such an estimator has high precision A good estimator has small standard error and small bias (or no bias at all) The following pictures represent different estimators with different bias and efficiency Assume that the true population parameter is the point (0,0) in the middle of the picture STA 291 Spring 2010 Lecture 12

Bias and Efficient Note that even an unbiased and efficient estimator does not always hit exactly the population parameter. But in the long run, it is the best estimator. STA 291 Spring 2010 Lecture 12

Point Estimators of the Mean and Standard Deviation Sample mean is unbiased, consistent, and (often) relatively efficient for estimating μ Sample standard deviation is almost unbiased for estimating population standard deviation No easy unbiased estimator exists Both are consistent STA 291 Spring 2010 Lecture 12