Notes

Question 1

What do the lines ‘in the box’ show as part of a box plot?

Answer 1

The median and the two quartiles (1st and the 3rd)

Question 2

What is the risk ratio?

Answer 2

The risk ratio is the probability of an event occurring in an exposed group compared to the probability occurring in a non-exposed group.

Question 3

Which studies commonly use the odds ratio as a measure of association? [1]

Answer 3

Odds ratios are most frequently used in case-control studies, but may also be used in cross-sectional studies.

Question 4

What is a type 2 error?

Answer 4

The failure to reject the null hypothesis when it is false
- false negative

Question 5

What is the attributable risk?

Answer 5

The attributable risk is another measure of association, which is defined as the amount of proportion of disease incidence (disease risk) that can be attributed to the exposure.

Question 6

What is a type 1 error?

Answer 6

The incorrect rejection of the null hypothesis when it is true
- false positive

Question 7

What is the main disadvantage of cross-sectional studies?

Answer 7

Cross-sectional studies do not show causality or account for confounding factors which may also influence the data. Therefore, associations identified may be difficult to interpret.

Question 8

How do you calculate relative risk?

Answer 8

The risk of poor outcomes in the intervention group divided by the risk of poor outcomes in the control group.

1

Question 9

When do you use a Mann-Whitney U test? [1]

Answer 9

Non-parametric data, two groups

Question 10

When do you use a Wilcoxon signed rank test? [1]

Answer 10

Non-parametric data, one group

Question 11

When do you use a Kruskal-Wallis test? [1]

Answer 11

Non-parametric data, more than 2 groups

Question 12

Non-parametric; more than two groups

Paired T-test
Kruskal Wallis Test
Mann Whitney U test
Wilcoxon test

Answer 12

Non-parametric; more than two groups

Paired T-test
Kruskal Wallis Test
Mann Whitney U test
Wilcoxon test

Question 13

Non-parametric; two groups

Paired T-test
Kruskal Wallis Test
Mann Whitney U test
Wilcoxon test

Answer 13

Mann Whitney U test

Question 14

Non-parametric; one group

Paired T-test
Kruskal Wallis Test
Mann Whitney U test
Wilcoxon test

Answer 14

Non-parametric; one group

Paired T-test
Kruskal Wallis Test
Mann Whitney U test
Wilcoxon test

Question 15

Which tests do you use for parametric [3] and non-paremetric [3]

		one group
		two groups
		multiple groups

Answer 15

Question 16

When do you use Fisher’s exact test instead of Chi-Squared? [1]

Answer 16

When 2x2 table has date with data with < 5 in it

Question 17

If you want to measure a binary outcome measured at two time points with a group of indivuals, which test do you use? [1]

Answer 17

McNemars test

Question 18

Describe how you calculate risk ratio [1]. e.g. using

Answer 18

Question 20

When would you use the following types of regression:

• Ordinal logistic
• Poission
• Tobit
• Cox’s proportional hazards

Answer 20

Question 21

How do you calculate the Specificity of a result? [1]

Answer 21

True Negative / (True Negative + False Positive)

95+255 = 350 / (350 + 45)

Question 22

How do you calculate sensitivity? [1]

Answer 22

True positive / (true positive + false negative )

Question 23

How do you calculate the PPV of a result? [1]

Answer 23

True +ve / (TP + FP)

Question 24

Negative predictive value?

Answer 24

TN / (TN + FN)

Question 25

What is specificity? [1]

Answer 25

**Specificity is the chance of someone without the disease testing negative.** e.g. A 68-year-old woman is being investigated for Helicobacter pylori infection. She is told that the specificity of the test is 89.8%. - **89.8% of patients without H. pylori test negative** ## Footnote Work out by: TN / (TN + FP)

Question 26

A question asks you to compare % and / or proportions - which test should you use? [1]

Answer 26

**Chi-squared test** - used to compare proportions or percentages e.g. compares the percentage of patients who improved following two different interventions

Question 27

*Patients were randomly allocated to either receive the new NRT or a placebo inhaler. The frequency of their cravings was assessed after a two-week period using the following scale: 'Never', 'Rarely', 'Sometimes', 'Often', 'Always'.* Test used?

Answer 27

**Mann-Whitney U test** - compares ordinal, interval, or ratio scales of unpaired data

Question 28

*compares the percentage of patients who improved following two different interventions* Which test? [1]

Answer 28

**Chi-squared test**

Question 29

How do you work out NNT? [+] e.g. A study looks at adding a new antiplatelet drug in addition to aspirin to patients who've had a stroke. One hundred and seventy patients are enrolled for the study with 120 receiving the new drug in addition to aspirin and the remainder receiving just aspirin. After 5 years 18 people who received the new drug had a further stroke compared to 10 people who just received aspirin. What is the number needed to treat?

Answer 29

1. Work out absolute risk reduction: Control event rate - experimental event rate 2. 1/absolute risk reduction Control event rate = 10 / 50 = 0.2 Experimental event rate = 18 / 120 = 0.15 Absolute risk reduction = 0.2 - 0.15 = 0.05 Number needed to treat = 1 / 0.05 = 20

Question 30

What does a Kaplein-Meier survival plot show? [1]

Answer 30

The Kaplan-Meier survival plot displays estimates of decreasing survival with time after an event

Question 31

*non-parametric & compares two sets of observations on a single sample, e.g. a 'before' and 'after' test on the same population following an intervention* ?

Answer 31

Wilcoxon signed-rank test

Question 32

How do you calculat variance of a sample? [1]

Answer 32

Variance = **square of standard deviation** | Variance is a measure of the spread of scores away from the mean

Question 33

What is the relationship between standard deviation and variance? Standard deviation = mean x variance Standard deviation = mean - variance Standard deviation and variance are the same Standard deviation is the square of variance Standard deviation is the square root of variance

Answer 33

What is the relationship between standard deviation and variance? Standard deviation = mean x variance Standard deviation = mean - variance Standard deviation and variance are the same Standard deviation is the square of variance **Standard deviation is the square root of variance**

Question 34

*compares a group with a disease to a group without, looking at past exposure to a possible causal agent for the condition*

Answer 34

Case control

Question 35

*compares two sets of observations on a single sample, e.g. a 'before' and 'after' test on the same population following an intervention*

Answer 35

Wilcoxon signed-rank test

Question 36

Type II error = ? [1]

Answer 36

**Type II error - the null hypothesis is accepted when it is false**

Question 37

Likelihood ratio for a positive test result = []

Answer 37

Likelihood ratio for a positive test result = **sensitivity / (1 - specificity)**

Question 38

What does the power of the study equate to? [1]

Answer 38

Power = **1 - the probability of a type II error**

Question 39

How do you calculate the liklihood ratio of a positive [1] and negative [1] event?

Answer 39

Liklihood ratio of positive event: - **Sensitivity / (1-specificity)** Liklihood ratio of negative event: - **(1-Sensitivity) / specificity**

Question 40

What type of calculation is used for a cohort study [1] and case-control study? [1]

Answer 40

Cohort: **relative risk** Case control: **odds ratio**

Question 41

How do you calculate standard error? [1] ## Footnote ```

Answer 41

STD / Sqr(number of participants)