Chi square analysis Just when you thought statistics was over!!

More statistics…  Chi-square is a statistical test commonly used to compare observed data with data we would expect to obtain according to a specific hypothesis.  For example, if, according to Mendel's laws, you expected 10 of 20 offspring from a cross to be male and the actual observed number was 8 males, then you might want to know about the "goodness to fit" between the observed and expected.

Hmmmmm…  Were the deviations (differences between observed and expected) the result of chance, or were they due to other factors?  How much deviation can occur before you, the investigator, must conclude that something other than chance is at work, causing the observed to differ from the expected?

Null hypothesis  The chi-square test is always testing what scientists call the null hypothesis, which states that there is no significant difference between the expected and observed result.

Chi Square x 2 =  ( O - E ) 2 E The formula…. Just get it over with already!!

Sample problem  Suppose that a cross between two pea plants yields a population of 880 plants, 639 with green seeds 241 with yellow seeds.  You are asked to propose the genotypes of the parents.  Your hypothesis is that the allele for green is dominant to the allele for yellow and that the parent plants were both heterozygous for this trait.  If your hypothesis is true, then the predicted ratio of offspring from this cross would be 3:1 (based on Mendel's laws) as predicted from the results of the Punnett square

GreenYellow Observed (o) Expected (e) Deviation (o - e)-2121 Deviation 2 (o - e) d 2 /e x 2 = d 2 /e = Chi Square x 2 =  ( O - E ) 2 E

So what does mean?  Figure out your Degree of freedom (dF)  Degrees of freedom can be calculated as the number of categories in the problem minus 1.  In our example, there are two categories (green and yellow); therefore, there is 1 degree of freedom.

Now that you know your dF…  Determine a relative standard to serve as the basis for accepting or rejecting the hypothesis.  The relative standard commonly used in biological research is p >  The p value is the probability that the deviation of the observed from that expected is due to chance alone (no other forces acting).  In this case, using p > 0.05, you would expect any deviation to be due to chance alone 5% of the time or less.

Conclusion  Refer to a chi-square distribution table  Using the appropriate degrees of 'freedom, locate the value closest to your calculated chi-square in the table.  Determine the closest p (probability) value associated with your chi-square and degrees of freedom.  In this case ( X 2 =2.668), the p value is about 0.10, which means that there is a 10% probability that any deviation from expected results is due to chance only.

Degrees of Freedom (df) Probability (p) NonsignificantSignificant

Step-by-Step Procedure for Chi-Square  1. State the hypothesis being tested and the predicted results.  2. Determine the expected numbers (not %) for each observational class.  3. Calculate X 2 using the formula.  4. Determine degrees of freedom and locate the value in the appropriate column.  5. Locate the value closest to your calculated X 2 on that degrees of freedom (df) row.  6. Move up the column to determine the p value.  7. State your conclusion in terms of your hypothesis.

Analysis  If the p value for the calculated X 2 is p > 0.05, accept your hypothesis. 'The deviation is small enough that chance alone accounts for it. A p value of 0.6, for example, means that there is a 60% probability that any deviation from expected is due to chance only. This is within the range of acceptable deviation.

 If the p value for the calculated X 2 is p < 0.05, reject your hypothesis, and conclude that some factor other than chance is operating for the deviation to be so great. For example, a p value of 0.01 means that there is only a 1% chance that this deviation is due to chance alone. Therefore, other factors must be involved.

Chi Square x 2 =  ( O - E ) 2 E 100 Flips of a coin Contingency table Heads Tails O E ( ) 2 50 ( ) ( 10 ) 2 50 ( 10 ) = = = = = 4.00 df = 1

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Distribution of colors….or so they say..… hmmmmmmm