Statistic for the day: Most popular street name in US: Second or 2 nd Number of Second (or 2 nd ) streets: Assignment: Read Sections 5.1, 5.2 Exercises: pp #1, 2, 3, 5 Source: 1990 Census These slides were created by Tom Hettmansperger and in some cases modified by David Hunter

Likely New Hampshire voters: Did the speech Howard Dean gave to his supporters after the Iowa Caucuses raise serious doubts in your mind that he can be a good candidate for president, or did it not affect your view of him? Gallup Poll Yes, raised serious doubts No, did not have an effect No opinion 30%69%1%

So in our example, if the sample size is 1600, then the square root is 40 and so the MARGIN OF ERROR is: Or 2.5 % And we report 30% + 2.5% 1/40 = 0.025

Summary: We have a simple random sample from the population of telephone owners who are likely voters in the NH primary. We have a simple random sample from the population of telephone owners who are likely voters in the NH primary. We find the percentage from our sample. We find the percentage from our sample. The MARGIN OF ERROR is 1 divided by the square root of the sample size. The MARGIN OF ERROR is 1 divided by the square root of the sample size. For 1600 the MARGIN OF ERROR is.025. For 1600 the MARGIN OF ERROR is.025.  Hence we report: PERCENTAGE The margin of error does not depend on the population size, only on the sample size! The margin of error does not depend on the population size, only on the sample size!

The Morning After Pill The Morning After Pill YesNo Not sure 59.1%37.1%3.8% Do you think that the ‘morning-after’ contraceptive pill should be available over the counter? USA Today call-in poll (

Pie Chart for Contraceptive Poll

The responding group is not representative of any larger group! Opinions reflect only those of the people who decide to respond. These polls are unscientific and worthless. READ P61 OF THE TEXT BOOK. What Is Wrong With the Poll?

Volunteer response vs. volunteer sample Contraceptive call-in poll? Volunteer sample! 1936 Literary Digest poll? Volunteer response! Which is worse? Volunteer sample!

Do you have a tattoo? YesMenNoMenYesWomenNoWomen 11%89%13%87% Based on: 37 men 77 women Stat100.2 F01

Pie Chart: Tattoos

What is the margin of error for the different percentages? What should you report? Example: men who answer yes 11% N = 37 Margin of error =.16 or 16% Report: 11% + 16% So there is a huge margin of error and the 11% is fairly uncertain.

What Is Wrong With the tattoo Survey? What is the target population? What sort of sample do we have?

Sampling methods (Simple) random sampling (Simple) random sampling Stratified random sampling Stratified random sampling Cluster sampling Cluster sampling Systematic sampling Systematic sampling Bad: Haphazard or convenience sampling (as in tattoo survey) Bad: Haphazard or convenience sampling (as in tattoo survey)

Stratified random sampling Divide population into subgroups, or strata Divide population into subgroups, or strata From each stratum, select a random sample From each stratum, select a random sample Example: Select a random sample from each of four groups of students (in-state non-minority, in-state minority, out-of-state non-minority, out-of-state minority) to ensure adequate representation of each group.

Cluster sampling Divide population into subgroups, or clusters Divide population into subgroups, or clusters Select a random sample of clusters Select a random sample of clusters Measure individuals within selected clusters according to some plan Measure individuals within selected clusters according to some plan Example: To study high schoolers, first take a random sample of schools and then look in depth at all students in selected schools

Systematic sampling From a list of individuals in the population, select every k th individual From a list of individuals in the population, select every k th individual Example: Does anyone know the origin of the word “decimate”?