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The Chi Square Test

The Chi Square Test. “The Goodness Fit” Test. Purpose. Used in genetics to determine how close to the expected ratio your data is Determines if the experiment was “significant” and not just due to chance Ex: Mendel crossed Tt x Tt (tall) plants Expected ratio- 3:1

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The Chi Square Test

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  1. The Chi Square Test “The Goodness Fit” Test

  2. Purpose • Used in genetics to determine how close to the expected ratio your data is • Determines if the experiment was “significant” and not just due to chance • Ex: Mendel crossed Tt x Tt (tall) plants • Expected ratio- 3:1 • Data: 290 Tall and 110 short • Pretty close... but how close?!

  3. Formula Observed = what your experiment showed Expected = what the experiment SHOULD have shown

  4. Find the expected by multiplying the TOTAL observed by the EXPECTED proportion • Expected should be 3:1 Tall: Short • (400)(.75) = 300 • (400)(.25) = 100 • You do the math!!

  5. The X² is the chi square value! Add!

  6. df = Degrees of freedom • # of categories/phenotypes minus 1 • p = Significance level • Percentage in decimal form • We will use p = 0.05 which means there is a 5% chance that the data is false • Or.. 95% that its true!

  7. Our example: Tall plants and Short plants df = 1 X² = 1.33 Find 0.5 and df 1. - 3.841 is the critical value If your X² is LOWER than this value, then you can ACCEPT your hypothesis (yaay!) If your X² is HIGHER than this value, then you have to REJECT your hypothesis

  8. Point?? • Since the critical value in the table was 3.841 and our value was 1.33… • Its lower… • We can accept our hypothesis! • It was “good enough!”

  9. Practice • Mendel: RrYy x RrYy (Round, Yellow) • Expected Outcome: • 9/16 Round, Yellow • 3/16 Round, Green • 3/16 Wrinkled, Yellow • 1/16 Wrinkled, Green

  10. df? Critical Value? Accept or Reject data??

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