Chapter 5 Sampling and Surveys 5.1 Samples, Good and Bad

Bias Samples Sample is a small group from a larger population. Design flaw of a statistical study that systematically favors certain outcomes. Systematically favoring one outcome over another Samples Sample is a small group from a larger population. Representative samples have the same characteristics of the larger population

BIASED SAMPLES: Voluntary Response Sample Individuals choose to become part of the sample by responding to a general request. Very biased in that many reply with extreme or strong opinions on the subject matter. Not a representative sample since not all people would choose to respond. EX: Call-in, write-in, putting out a flyer asking for participants, etc.

BIASED SAMPLE: Convenience Sample Selection of individuals that are easily obtained. Not a representative sample. Example: choosing your 10 friends that you happen to see that day Selecting the first 10 names on a list of participants

Unbiased Sample Simple Random Sample (SRS) Sample of size n from a population chosen in such a way that: Every individual of the population has the same chance of being selected Every possible sample of size n has the same chance of being selected.

Example: 4 Students need to be selected to represent the class in Statistics Conference in Cancun during Spring Break. Put everyone’s name in hat, mix it up, and select 4 names. Put all the boy’s names in one hat and all the girl’s names in a second hat. Randomly select 2 boys and 2 girls. X

How to take a SRS: Table of Random Digits (table B)- A long string of digits (0-9) with the following properties: Each digit in the table is equally likely to be any of the 10 digits The entries are independent of each other. So knowing one digit doesn’t affect what the next one is.

HOW TO USE THE TABLE: 1) LABEL * Label each individual in your population with a # * Each # given must have the same amount of digits 2) TABLE * Go to the table, read across in groups that are the same size as the numbers you assigned

How to write instructions for SRS: Label each (person) with # (01) -- ____ Read across TRD, every digits represents a (person) . Ignore #’s , and repeats The first numbers are our sample

EXAMPLE: Take a sample 5 from the following list. Start at line 131 in the table. Smith Jones Holloway DeNizzo David Adams Schaefer Gray Capito Meyers Gingrich Card Dietrich Moreland Hall Walsh Whitter Jordan

Instructions Label each person with #01-18 Read across the TRD, every 2 digits represents a person Ignore 19-99,00 and repeats The first 5 numbers are our sample

First, label each person with a number, starting at 01 EXAMPLE: First, label each person with a number, starting at 01 01 Smith 07 Jones 13 Holloway 02 DeNizzo 08 David 14 Adams 03 Schaefer 09 Gray 15 Capito 04 Meyers 10 Gingrich 16 Card 05 Dietrich 11 Moreland 17 Hall 06 Walsh 12 Whitter 18 Jordan

* Next, read across table every 2 digits and circle numbers between 01 and 18, ignoring repeats, til you find 5 numbers. Line 131: 05|00|7 1|66|32| 81|19|4 1|48|73| 04|19|7 8|55|76| 45|19|5 9|65|65 Line 132: 68|73|2 5|52|59| 84|29|2 0|87|96| 43|16|5 9|37|39 31|68|5 9|71|50 Line 133: 45|74|0 4|18|07| 65|56|1 3|33|02 |07|05|1

Find the people you selected by matching up the numbers selected: EXAMPLE: Find the people you selected by matching up the numbers selected: 01 Smith 07 Jones 13 Holloway 02 DeNizzo 08 David 14 Adams 03 Schaefer 09 Gray 15 Capito 04 Meyers 10 Gingrich 16 Card 05 Dietrich 11 Moreland 17 Hall 06 Walsh 12 Whitter 18 Jordan