6 - Analysis of Variation Oneway

Question 1

What is the difference between t tests and ANOVA?

Answer 1

• t tests = comparing the means of 2 groups

- ANOVA = comparing the means of > 2 groups

Question 2

What is the problem w/ doing multiple t tests instead of ANOVA?

Answer 2

Multiple t tests increases the chance of type 1 error (that the test will say a result is significant when it in fact is not)

Question 3

With ANOVA, what 2 things are we concerned with?

Answer 3

1) Variability within each group

2) Variability between the groups

Question 4

What is the formula for F value?

Answer 4

F = variance BETWEEN sample means / variance among individuals WITHIN each sample

Question 5

What is the difference between SD and variance?

Answer 5

Variance = SD^2

Question 6

What is the difference between SSE and SSW?

Answer 6

• Same thing

- One is sum of squares error and one is sum of squares within groups

Question 7

How do you calculate df?

Answer 7

of groups minus 1

Question 8

What are you comparing for between group variance?

Answer 8

The means of each group

Question 9

What are you comparing for within group variance?

Answer 9

How each variable in a group compares to the mean of that group

Question 10

What are some assumptions for ANOVA one way?

Answer 10

• Simple random samples reduce bias
• We have independent samples, one from each of k populations
• For dependent measures (repeated measures on the same subject) a repeated measures ANOVA model is available
• The population from which the simple random samples are drawn must be normally distributed
• The sigma squared for each population must be equal (not significantly different, even when group means are different)

Question 11

How do you know which critical F point to pick from the chart?

Answer 11

First number will be df for group 1, second number will be df for group 2

Question 12

How must the calculated F value compare to the critical F value to be considered significantly different?

Answer 12

Calculated F value must be greater than critical F value

Question 13

What does a significant F value tell you w/ ANOVA (when there are more than 2 groups)?

Answer 13

Tells you that at least 2 of the groups differ; doesn’t tell you which ones

Question 14

What is the difference between SSB and SST?

Answer 14

• Same thing

- One is sum of squares between and the other is sum of squares treatment

Question 15

For an example w/ 3 different treatment groups and N = 43 patients, what would be the df for between tx and within tx?

Answer 15

• Between tx = # of groups minus 1 = 3-1 = 2

- Within tx = # of patients minus # of groups = 43-3 = 3

Question 16

When using the F distribution table, if a number you need isn’t on the table do you round up or down? Why

Answer 16

Round down so that it makes it that much harder to pass the test (being more conservative)
- So if you have 55, round down to 50 instead of up to 60