# Statistics 3 Contingency Tables AS AQA

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• Created by: Brandon
• Created on: 29-05-13 12:53
• 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