Statistics 3 Contingency Tables AS AQA

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  • Created by: Brandon
  • Created on: 29-05-13 12:53
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  • Contingency Tables
    • A Contingency Table shows the frequencies of two or more variables simultaneously.
      • X2 Distribution
        • X2 = SIGMA (O-E)2 */* E
          • Compare with critical value. Degrees' of Freedom
            • An (m x n) Rows-1 x Column-1 i.e. Rows=2 Column is 3, 2-1x3-1 = 2
              • At 5% X2 Crit Value = 5.991
        • Only work if Os are frequencies + Es are reasonably large i.e. over 5
    • O - Original Value
    • E - Expected Value
      • Calculated after the O.
        • E = Row Total x Column total */* Grand Total
    • Ho - Variable is Independent
      • H1 - Variable not Independent
        • Reject Ho if X2 larger than Critical Value
    • Small Expected Values
      • Can lead to a bad Critical Value so combining classes is needed to increase the value of E
        • E should be Large
          • Classes need to be combined so data still remains a contingency table
          • In order to interpret the conclusions, classes with small E's should be combined with similar classes.
    • Yate's Correlation
      • 2X2 tables
        • So find out X2 but like this -->
        • Sigma (O-E/-0.5)2 / E
          • I.E. when O+E=x when x is 4 then 4-0.5 is 3.5
    • ONE TAILED

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