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Statistics

Statistics. AP Biology. Descriptive Statistics. .used to summarize data .calculate averages: mean, medium, and mode .measure variation: range, standard error, standard deviation. Chi-Squared Test. .Compares OBSERVED results with EXPECTED results

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Statistics

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  1. Statistics AP Biology

  2. Descriptive Statistics .used to summarize data .calculate averages: mean, medium, and mode .measure variation: range, standard error, standard deviation

  3. Chi-Squared Test • .Compares OBSERVED results with EXPECTED results • .The answer tells if the results are SIGNIFICANTLY different • is this difference because of chance or some other factor • If the answer is 0, then the observed and the expected are the same • If the answer is 100, then the observed did not occur by chance and the data is significantly different • The answer will usually be in-between 0 and 100 (a 95% answer is acceptable) • We use a statistical table to determine the results

  4. Chi-Squared Test Formula • Where: • X2 = test statistic • Σ= sum of • 0= observed value • E= expected value

  5. Example Eye Color in Fruit Flies Red eye= dominant R Brown eye= recessive r We EXPECT a cross between Rr (heterozygous dominant, red eye) and rr (homozygous recessive, brown eye) to produce ½ Rr and ½ rr. We OBSERVED 55 Rr (red eye) and 45 rr (brown eye) What will the X2 statistic be? 1.Make a contingency table with all the values 2.Calculate values

  6. Fruit Fly eye example continued Now Plug the values into the Chi-Squared formula .5+.5 = 1

  7. Fruit Fly Eyes Cont. Once you have an answer you must now determine the DEGREES OF FREEDOM (df) = number of E values – 1 2-1 =1 Once you have found df, use the given Chi-Square Table to determine if the OBSERVED values differ from the EXPECTED VALUES Where: Table= critical values for x2 P = probability At: P=.05

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