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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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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
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
simple random sample srs
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
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
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
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
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
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