test for significant differences t- tests. t- test t-test – is a statistical test that compares...
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Test for Significant Differences
T- Tests
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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.
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T- Test
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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.
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T-Tables
• Degrees of freedom (df): Sum of sample sizes minus 2
• Probability Values (p-value) – Probably that the difference occurred by chance alone
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Degrees of Freedom
• What is our sample size?• 20• What is the degrees of
freedom?• 20 – 2 =18
Boys Scores
Girls Scores
1 60 98
2 62 42
3 68 88
4 70 92
5 63 38
6 65 56
7 65 95
8 58 92
9 64 50
10 63 89
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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.
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Null Hypothesis H0
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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
• By convention, if P > 0.05 you can conclude that the result is not significant (the two samples are not significantly different).
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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.
– If P < 0.01 you say the result is highly significant and you can be more confident you have found a true effect.
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P-Value Summary
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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.
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T-Test & Excel
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T-Test & Excel
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T-Test & Excel
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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.