RANDOM SAMPLING

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# RANDOM SAMPLING - PowerPoint PPT Presentation

RANDOM SAMPLING. POPULATION- the entire group of individuals that we want information about SAMPLE- the part of the population that we actually examine in order to gather information about the population. Population vs. Sample. Gallup finds unemployment at 10.1% in September

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### RANDOMSAMPLING

POPULATION- the entire group of individuals that we want information about

• SAMPLE- the part of the population that we actually examine in order to gather information about the population
Population vs. Sample
• Gallup finds unemployment at 10.1% in September
• The findings were based on more than 18,146 phone interviews (random digit dialing) with U.S. adults aged 18 and older
• POPULATION: all U.S. adults ages 18 and older who have telephones
• SAMPLE: 18,146 U.S. adults with telephones
SIMPLERANDOM SAMPLE (SRS)
• A SRS (of size n) is selected from a population in a way that every different possible sample of size n has the same probability of being selected
• Select an SRS of size 3 from our AP Stat course
• All possible groups of 3 students have the same probability of being selected as any other group of 3 students
• Not an SRS: Split students into males and females. Randomly select one female and two males.
Constructing an SRS
• Create a list of the sampling frame
• Give each object a number
• Use a random number table to select sample
• An Example: Randomly select 4 students from our AP Stat class
• List all students available for selection
• Give each student a number
• Since the largest number of students that we can select only has two digits, we will take numbers in groups of 2 (start on line 130 of Table B)
Generating Random Numbers on the TI Graphing Calculator
• 7  Math, rand, enter (change random seed #)
• Math >>> 5 (PRB)
• randInt (min,max)
• Note: If a number is selected more than once, ignore it
Stratified Random Sample
• The population is divided in to at least two distinct groups (strata).
• A simple random sample of a certain size is drawn from each strata.
• Often, the size is chosen by using the actual percentage of occurrence in the overall population
• Example: Divide class into girls and boys and randomly select from each gender in the proportion that they occur in the population (class).
Cluster Sample
• Divide the population into subgroups (clusters)
• Select clusters at random
• Include all individuals in the selected clusters in your study
• Example: Randomly select homerooms and then include all students in the homerooms in the sample
• In class, randomly select a row, include all students in the row to be in the sample
• Multi-Stage Cluster Sample
Systematic Sampling
• The elements in the population should already be arranged in some natural sequential order
• Choose every kth element in the population to be part of your sample
• List the students in AP Statistics alphabetically
• Number the students
• Randomly select a student
• Choose every kth student depending on the size of your sample
• K = N/n = ________ Round down. Choose every _______

student