ANOVA Flashcards

1
Q

What does ANOVA stand for, and what is its purpose?

A

ANalysis Of VAriance (ANOVA) tests for differences in means across three or more groups.

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2
Q

Why shouldnโ€™t we use multiple t-tests for comparing more than two groups?

A

Multiple t-tests increase the Type I error rate, raising the probability of falsely rejecting the null hypothesis.

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3
Q

If there are 5 groups, how many pairwise comparisons would there be?

A

10

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4
Q

What is a Type I error?

A

Rejecting the null hypothesis (๐ป0) when it is actually true.
Probability = ๐›ผ (significance level, e.g., 0.05).

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5
Q

What is a Type II error?

A

Failing to reject the null hypothesis (๐ป0) when it is actually false.
Probability = ๐›ฝ.

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6
Q

How does performing multiple t-tests increase the Type I error rate?

A

For ๐‘˜ comparisons, the probability of at least one Type I error is:1โˆ’(1โˆ’ฮฑ)^k

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7
Q

How does the Bonferroni correction control the Type I error rate?

A

Divide the significance level by the number of comparisons:
๐›ผ๐‘˜=๐›ผ/๐‘˜

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8
Q

What are the null and alternative hypotheses for ANOVA?

A

H0:ฮผA=ฮผB=ฮผC (means are equal)

๐ป1: at least one mean is different.

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9
Q

What does the ANOVA test statistic (๐น) compare?

A

F= Variabilitybetweengroups / Variabilitywithingroups
โ€‹

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10
Q

What is the formula for the between-group variance (๐‘ ๐บ2)?

A

๐‘ ๐บ2 = sum of squares between SSG / (k-1)

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11
Q

What is the formula for the within-group variance (๐‘ ๐‘Š2)?

A

๐‘ ๐‘Š2 = sum of squares within (SSW) / (ntot -k)
ntot = total sample size,
๐‘˜ = number of groups.

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12
Q

What does a large F-statistic indicate in ANOVA?

A

A large F-statistic means between-group variability is large compared to within-group variability, suggesting significant differences between group means.

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13
Q

How do you interpret the ANOVA table?

A

Between groups: Large Mean Square indicates group differences.

Residuals (within groups): Represents random variability.

Large ๐น-value + small ๐‘-value = reject ๐ป0

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14
Q

What degrees of freedom are used in ANOVA?

A

df1=kโˆ’1 (between groups)

๐‘‘๐‘“2=๐‘›๐‘ก๐‘œ๐‘กโˆ’๐‘˜ (within groups)

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15
Q

What does a small p-value in ANOVA indicate?

A

It indicates strong evidence that at least one group mean is significantly different.

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16
Q

What is the test statistic in ANOVA?

A

The F-statistic: SG2 / SW2

17
Q

If the ANOVA test is significant, what should you do next?

A

Perform a post-hoc test (like Tukeyโ€™s test) to identify which groups differ.

18
Q

What assumptions does ANOVA rely on?

A

Normality of data within groups.

Homogeneity of variances across groups.

Independence of observations.

19
Q

What should you do if ANOVA assumptions are violated?

A

Use a non-parametric alternative like the Kruskal-Wallis test.

20
Q

Given ๐น0=5.263, ๐‘‘๐‘“1=2, ๐‘‘๐‘“2=31499df2=31499, and a critical value of 2.996, should you reject ๐ป0?

A

Yes, reject ๐ป0 because:

๐น0=5.263 > 2.996 (critical value)

๐‘-value=0.00518 < 0.01.

There is strong evidence that at least one mean is different.

21
Q

What does the ๐‘-value of 0.00518 mean in this context?

A

It is highly unlikely to obtain an F-statistic this extreme (๐น0=5.263) if ๐ป0 (all means are equal) were true.

22
Q

After rejecting ๐ป0, what should you do next?

A

Perform a post-hoc test like Tukeyโ€™s HSD to determine which groups are significantly different.

23
Q

Why is the adjusted ๐‘-value (p adj) used in Tukeyโ€™s test?

A

It controls for the family-wise error rate by adjusting the significance level for multiple comparisons.

24
Q

What is the family-wise error rate, and why is it important?

A

It is the overall probability of making at least one Type I error when conducting multiple comparisons. It is controlled by adjusting ๐›ผ

25
Q

What are the key assumptions for a valid ANOVA test?

A

Independence of data within and between groups.

Equal variances across groups (homogeneity of variance).

Normal distribution of data within groups.

26
Q

How can you check the assumption of equal variances?

A

compare the standard deviation

Rule of thumb: the largest ๐‘  should be no more than twice the smallest.

Alternatively, use Leveneโ€™s test in R

27
Q

How do you test for normality in ANOVA?

A

Plot histograms or Q-Q plots.

Perform a Shapiro-Wilk test in R.

28
Q

What non-parametric test can be used if the ANOVA assumptions are violated?

A

Kruskal-Wallis test (for non-normal data).

It assumes:
Independent data.

Similar shape of distributions across groups.

29
Q

Why is practical significance different from statistical significance?

A

Statistical significance means a result is unlikely due to chance, while practical significance considers if the difference is meaningful in real-world terms.

30
Q

What are the steps in hypothesis testing?

A

State ๐ป0 and ๐ป1

Collect data.

Select a test (e.g., ANOVA, t-test, etc.).

Set ๐›ผ (e.g., 0.05).

Find the reference distribution (e.g., F-distribution).

Calculate test-statistic & p-value.

Make a decision (reject/fail to reject ๐ป0).

Interpret the result.

31
Q

Why is the F-distribution used in ANOVA?

A

It compares the variance between groups to the variance within groups to test if the means differ.

32
Q

What does a large F-statistic imply?

A

Large differences between group means relative to the within-group variability, suggesting significant group differences.

33
Q

What does failing to reject ๐ป0 mean?

A

There isnโ€™t enough evidence to conclude that any group means are different.