1. AP Biology

2. Chi-Square Purpose: To determine if a deviation from expected results is significant. (or was it just chance) Purpose: To determine if a deviation from expected results is significant. (or was it just chance) Definition: A measure of the deviation between observed & expected experimental results. Definition: A measure of the deviation between observed & expected experimental results.

3. Equation X 2 = Chi Square ∑ = Summation Fo = Observed frequency Fe = expected frequency

4. Example of Chi Square Flip a coin 100 times. Record the number of heads & tails. Flip a coin 100 times. Record the number of heads & tails. Plug data into formula. Plug data into formula. Data: 48 heads 52 tails Data: 48 heads 52 tails heads4850-24 4/50 =.08 tails525024 o e o-e (o-e) 2 (o-e) 2 /e ∑ =.08 +.08 =.16 X 2 =.16 X 2 = ∑ (48 – 50) 2 / 50 + (52 – 50) 2 / 50 ^heads ^tails ^heads ^tails

5. How to read the value Look on the hand-out chart. Find the “number of classes” column. Select the number of choices that your experiment resulted in. ( Heads or tails were the choices in this example, so there were two choices.) Move across the proper row to find your x 2 number. Read the number of times per 100 that chance alone would produce a deviation as large or larger. PROCESS the meaning of this in your brain. A small chi square value indicates that the variation is due to chance. A large chi square number means that the variation is statistically significant and that it is the result of some other factor.