1. Test for Significant Differences T- Tests

2. T- Test T-test – is a statistical test that compares two data sets, and determines if there is a significant difference between two data sets.

3. T- Test

4. Calculating T-Test To calculate a test test (t-values) you need the following information: – Mean – Standard deviation – Sample size You don’t need to calculate t-values – excel will do it for you.

5. T-Tables Degrees of freedom (df): Sum of sample sizes minus 2 Probability Values (p-value) – Probably that the difference occurred by chance alone

6. Degrees of Freedom What is our sample size? 20 What is the degrees of freedom? 20 – 2 =18 Boys Scores Girls Scores 16098 26242 36888 47092 56338 66556 76595 85892 96450 106389

7. What does it all mean? If your sample size is 10 and your t-value is 5.00, what is your probably (p-value) that the difference between two sets occurred by chance? Degrees of freedom (df) = 8 P-value is 0.001 which means there is a 0.1% chance that the difference occurred by chance. Thus the difference between the two data sets is probably statistically significant.

8. Null Hypothesis H 0

9. Significant Differences If you carry out a statistical significance test, such as the t-test, the result is a P value, where P is the probability that there is no difference between the two samples. – A. When there is no difference between the two samples: A small difference in the results gives a higher P value, which suggests that there is no true difference between the two samples

10. Significant Differences If you carry out a statistical significance test, such as the t- test, the result is a P value, where P is the probability that there is no difference between the two samples. B. When there is a difference between the two samples: – A larger difference in results gives a lower P value, which makes you suspect there is a true difference (assuming you have a good sample size). – By convention, if P < 0.05 you say the result is statistically significant.

11. P-Value Summary

12. P-value & Null Hypothesis In biology the critical probability (P-value) is usually taken as 0.05 (or 5%). This may seem very low, but it reflects the facts that biology experiments are expected to produce quite varied results. If P > 5% then the two sets are the same (i.e. accept the null hypothesis). – Null hypothesis is always A PREDICTION stating that there is no significant differences between data sets. If P < 5% then the two sets are different (i.e. reject the null hypothesis). – Null hypothesis is always A PREDICTION stating that there is no significant differences between data sets.

16. T-Test & Excel

19. Your turn. Day 2 Run a T-test comparing the data sets to test to see if you have a significant difference between snail shells in habitats with and without crabs.. Screen shot you excel calculations and insert them into a word document. In an organized table report your p- value, t-value, degrees of freedom. State your null hypothesis. State whether you accept of reject your null hypothesis. Provide a reason for your statement in d. Your reason should include your p- values. Provide a scientific reason for your results.