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Statistics

Statistics. Interval Estimation. STATISTICS in PRACTICE. Food Lion is one of the largest supermarket chains in the USA. Food Lion establishes a LIFO (Last in First Out) index for each of seven inventory pools : Grocery, Paper/Household, Pet Supplies, Health & Beauty Aids,

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  1. Statistics Interval Estimation

  2. STATISTICSin PRACTICE • Food Lion is one of the largest supermarket chains in the USA. • Food Lion establishes a LIFO (Last in First Out) index for each of seven inventory pools: Grocery, Paper/Household, Pet Supplies, Health & Beauty Aids, Dairy, Cigarette/Tobacco, and Beer/Wine.

  3. STATISTICSin PRACTICE • Using a 95% confidence level, Food Lion computed a margin of error of .006 for the sample estimate of LIFO index. • The interval from 1.009 to 1.021 provided a 95% confidence interval estimate of the population LIFO index. • This level of precision was judged to be very good.

  4. Interval Estimation • Population Mean: s Known • Population Mean: s Unknown • Determining the Sample Size • Population Proportion

  5. Margin of Error and the Interval Estimate • A point estimator cannot be expected to provide • the exact value of the population parameter. • An interval estimate can be computed by adding • and subtracting a margin of error to the point • estimate. Point Estimate +/- Margin of Error • The purpose of an interval estimate is to provide • information about how close the point estimate • is to the value of the parameter.

  6. Margin of Error and the Interval Estimate • The general form of an interval estimate of a • population mean is

  7. Interval Estimation of a Population Mean: sKnown • In order to develop an interval estimate of a population mean, the margin of error must be computed using either: • the population standard deviation s , or • the sample standard deviation s • sis rarely known exactly, but often a good estimate can be obtained based on historical data or other information. • We refer to such cases as the s known case.

  8. Interval Estimation of a Population Mean: sKnown There is a 1 - probability that the value of a sample mean will provide a margin of error of or less.

  9. Sampling distribution of 1 -  of all values Interval Estimation of a Population Mean: sKnown /2 /2 

  10. 1 -  of all values Interval Estimate of a Population Mean: sKnown Sampling distribution of /2 /2 interval does not include m  interval includes m interval includes m [------------------------- -------------------------] [------------------------- -------------------------] [------------------------- -------------------------]

  11. where: is the sample mean 1 -is the confidence coefficient z/2 is the z value providing an area of /2 in the upper tail of the standard normal probability distribution sis the population standard deviation nis the sample size Interval Estimate of a Population Mean: s Known • Interval Estimate ofm

  12. Confidence Levels • Values of for the Most Commonly Used Confidence Levels.

  13. Interval Estimate of a Population Mean: sKnown • Adequate Sample Size • In most applications, a sample size of n = 30 is • adequate. • If the population distribution is highly skewed or • contains outliers, a sample size of 50 or more is • recommended.

  14. Interval Estimate of a Population Mean: sKnown • Adequate Sample Size (continued) • If the population is not normally distributed but • is roughly symmetric, a sample size as small as • 15 will suffice. • If the population is believed to be at least • approximately normal, a sample size of less than • 15 can be used.

  15. D S Interval Estimate of Population Mean: Known • Example: Discount Sounds Discount Sounds has 260 retail outlets throughout the United States. The firm is evaluating a potential location for a new outlet, based in part, on the mean annual income of the individuals in the marketing area of the new location.

  16. D S Interval Estimate of Population Mean: Known • Example: Discount Sounds A sample of size n = 36 was taken; the sample mean income is $31,100. The population is not believed to be highly skewed. The population standard deviation is estimated to be $4,500, and the confidence coefficient to be used in the interval estimate is .95.

  17. D S 95% of the sample means that can be observed are within + 1.96 of the population mean .  Interval Estimate of Population Mean: Known The margin of error is: Thus, at 95% confidence, the margin of error is $1,470.

  18. D S Interval Estimate of Population Mean:Known Interval estimate of  is: $31,100 + $1,470 or $29,630 to $32,570 We are 95% confident that the interval contains the population mean.

  19. Interval Estimation of a Population Mean: s Unknown • If an estimate of the population standard deviation s cannot be developed prior to sampling, we use the sample standard deviation s to estimate s. • This is the sunknown case. • In this case, the interval estimate for mis based on the t distribution. • (We’ll assume for now that the population is normally distributed.)

  20. t Distribution • The t distribution is a family of similar • probability distributions. • A specific t distribution depends on a • parameter known as the degrees of freedom. • Degrees of freedom refer to the number of • independent pieces of information that go • into the computation of s.

  21. t Distribution • A t distribution with more degrees of freedom has • less dispersion. • As the number of degrees of freedom increases, • the difference between the t distribution and the • standard normal probability distribution becomes • smaller and smaller.

  22. t Distribution t distribution (20 degrees of freedom) Standard normal distribution t distribution (10 degrees of freedom) z, t 0

  23. t Distribution Table • For Areas in the Upper Tail

  24. The standard normal z values can be found • in the infinite degrees ( ) row of the • t distribution table. t Distribution • For more than 100 degrees of freedom, the • standard normal z value provides a good • approximation to the t value.

  25. t Distribution Standard normal z values

  26. Interval Estimation of a Population Mean:sUnknown • Interval Estimate  where: 1 - = the confidence coefficient =the t value providing an area of  in the upper tail of a t distribution with n - 1 degrees of freedom s = the sample standard deviation t/2 /2

  27. Interval Estimation of a Population Mean:sUnknown • Example: Apartment Rents A reporter for a student newspaper is writing an article on the cost of off-campus housing. A sample of 16 efficiency apartments within a half-mile of campus resulted in a sample mean of $650 per month and a sample standard deviation of $55.

  28. Interval Estimation of a Population Mean: sUnknown • At 95% confidence,  = .05, and /2 = .025. • t.025is based on n - 1 = 16 - 1 = 15 degrees of freedom. • In the t distribution table we see that t.025 = 2.131.

  29. Interval Estimation of a Population Mean: sUnknown

  30. Interval Estimation of a Population Mean: sUnknown • Interval Estimate We are 95% confident that the mean rent per month for the population of efficiency apartments within a half-mile of campus is between $620.70 and $679.30.

  31. Summary of Interval Estimation Procedures for a Population Mean Can the population standard deviation s be assumed known ? Yes No Use the sample standard deviation s to estimate s sKnown Case Use Use sUnknown Case

  32. Sample Size for an Interval Estimate of a Population Mean • Let E = the desired margin of error. • E is the amount added to and subtracted from the • point estimate to obtain an interval estimate.

  33. Sample Size for an Interval Estimate of a Population Mean • Margin of Error • Necessary Sample Size (rounding up to the next integer value)

  34. Sample Size for an Interval Estimateof a Population Mean • Necessary Sample Size, if  is Unknown • Use the estimate of the population standard deviation computed of previous studies as the planning value for . • Use a pilot study to select a preliminary sample. The sample standard deviation from the preliminary sample can be used as the planning value for .

  35. Sample Size for an Interval Estimateof a Population Mean • Use judgment or a “best guess” for the value of . For example, the range divided by 4 is often suggested as a rough approximation of the standard deviation and thus an acceptable planning value for .

  36. Sample Size for an Interval Estimateof a Population Mean Recall that Discount Sounds is evaluating a potential location for a new retail outlet, based in part, on the mean annual income of the individuals in the marketing area of the new location. Suppose that Discount Sounds’ management team wants an estimate of the population mean such that there is a .95 probability that the sampling error is $500 or less. How large a sample size is needed to meet the required precision?

  37. D S Sample Size for an Interval Estimate of a Population Mean At 95% confidence, z.025 = 1.96. Recall that = 4,500. A sample of size 312 is needed to reach a desired precision of + $500 at 95% confidence.

  38. Interval Estimationof a Population Proportion The general form of an interval estimate of a population proportion is

  39. The sampling distribution of plays a key role • incomputing the margin of error for this • interval estimate. • The sampling distribution of can be • approximated by a normal distribution • whenever np> 5 and n(1 – p) > 5. Interval Estimationof a Population Proportion

  40. Normal Approximation of Sampling Distribution • of Sampling distribution of 1 -  of all values Interval Estimationof a Population Proportion /2 /2 p

  41. Interval Estimationof a Population Proportion • Interval Estimate where:  is the confidence coefficient is the z value providing an area of  in the upper tail of the standard normal probability distribution is the sample proportion where:  is the confidence coefficient is the z value providing an area of  in the upper tail of the standard normal probability distribution is the sample proportion

  42. Interval Estimation of a Population Proportion • Example: Political Science, Inc. Political Science, Inc. (PSI) specializes in voter polls and surveys designed to keep political office seekers informed of their position in a race. Using telephone surveys, PSI interviewers ask registered voters who they would vote for if the election were held that day.

  43. Interval Estimation of a Population Proportion • Example: Political Science, Inc. In a current election campaign, PSI has just found that 220 registered voters, out of 500 contacted, favor a particular candidate. PSI wants to develop a 95% confidence interval estimate for the proportion of the population of registered voters that favor the candidate.

  44. where: n = 500, = 220/500 = .44, z/2 = 1.96 = .44 + .0435 Interval Estimation of a Population Proportion PSI is 95% confident that the proportion of all voters that favor the candidate is between .3965 and .4835.

  45. However, will not be known until after we have selected the sample. We will use the planning value p* for . Sample Size for an Interval Estimate of a Population Proportion • Margin of Error Solving for the necessary sample size, we get

  46. Sample Size for an Interval Estimateof a Population Proportion • Necessary Sample Size The planning value p* can be chosen by: 1. Using the sample proportion from a previous sample of the same or similar units.

  47. Sample Size for an Interval Estimateof a Population Proportion 2. Selecting a preliminary sample and using the sample proportion from this sample. 3. Use judgment or a “best guess” for the value of p*. 4. If none of the preceding alternatives apply, use a planning value of p* = .50.

  48. Sample Size for an Interval Estimateof a Population Proportion • Some Possible Values for p*(1 - p*)

  49. Sample Size for an Interval Estimate of a Population Proportion Suppose that PSI would like a .99 probability that the sample proportion is within + .03 of the population proportion. How large a sample size is needed to meet the required precision? (A previous sample of similar units yielded .44 for the sample proportion.)

  50. At 99% confidence, z.005 = 2.576. Recall that  = .44. A sample of size 1817 is needed to reach a desired precision of + .03 at 99% confidence. Sample Size for an Interval Estimate of a Population Proportion

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