EAB - T-Test And Chi Squared Tests

Question 1

what is the difference between a paired and unpaired t-test

Answer 1

paired = matched samples (ie the same thing or same person, measuring blood pressure in 30 people over a month)

unpaired = independent samples (ie height in girls vs height in boys)

Question 2

whats the function of a t-test?

Answer 2

T-tests compare TWO means

Question 3

what is a chi-squared test used for? How is it different to t test

Answer 3

Test for an association between 2 categorical variables

The t-test is used to compare means between two groups, typically for continuous data, to determine if there is a significant difference between them. Chi-square test, on the other hand, is used to determine if there is a significant association between two categorical variables. It’s used when data are frequencies or counts, rather than continuous values.

So t test compares difference, chi squared tests association

Question 4

what are 2 assumptions of a t-test

Answer 4

• continuous data, normally distributed
• variances/SD same

Question 5

what is chi squared test based on?

Answer 5

Based on the chi-squared distribution with n degrees of freedom where n is given by (no. of rows-1) x (no. of columns-1)

Question 6

what is the chi squared test equivalent to?

Answer 6

Equivalent to the z test for two proportions (where each variable has only 2 categories)

Question 7

what does a chi squared test give you?

Answer 7

Gives a P value but no direct estimate or confidence interval for the estimates

Question 8

What is the null hypothesis?

Answer 8

In the population where the samples come, there is no association between the two variables

Question 9

What are the 4 rationales of chi-squared tests?

Answer 9

• Calculates the expected frequencies if there were no association (i.e. null hypothesis is true)
• Compares the observed frequencies with these expected values
• If the observed frequencies are very different to the expected values this provides evidence that there is an association
• The test uses a formula based on the chi-squared distribution to give a P value

Question 10

What are the 4 assumptions of the chi-squared test?

Answer 10

• ‘Large sample’ test
• Rule of thumb for test to be valid:
  
  • at least 80% of expected frequencies must be greater than 5
• For 2x2 test this will be true if all frequencies >5
• If assumptions don’t hold, consider collapsing the table if multi- category, or using Fisher’s exact test

Question 11

what 3 things do you need to make sure when using a chi squared test?

Answer 11

use FREQUENCIES not percentages

works for all size tables

test is usually done with a computer programme