Stata Lecture 4

Question 1

How is proportion calculated for a single group?

Answer 1

(Number of participants in category)/(Total number of participants)

Question 2

How is proportion represented?

Answer 2

Number between 0 and 1

Question 3

What are the assumptions for calculating confidence interval?

Answer 3

That the data is normally distributed

There are at least 5 observations in each category

Question 4

How is the odds of having the disease calculated for a single group?

Answer 4

(Number of participants in disease category)/(Number of participants in other category)
OR
(Proportion of participants in disease category)/(Proportion of participants in other category)

Question 5

What is the relationship between proportion and odds?

Answer 5

The greater the proportion of people in the disease category, the greater the odds the you will have the disease
If proportion = 1, the odds = infinity

Question 6

What are the two methods of summarising a binary variable for a single group?

Answer 6

Proportion and Odds

Question 7

What would you use to compare groups in a case-control study?

Answer 7

Odds

Question 8

What is the exposure variable vs the outcome variable?

Answer 8

Exposure variable = intervention/control

Outcome variable = measured

Question 9

How is the risk difference calculated between two independent groups?

Answer 9

(Proportion with disease in intervention group) - (proportion with disease in control group)

Question 10

How is the risk ratio calculated between two independent groups?

Answer 10

(Proportion with disease in intervention group)/(Proportion with disease in control group)

Question 11

How is the odds ratio calculated between two independent groups?

Answer 11

(Odds of having disease in one group)/(Odds of having disease in the other group)

Question 12

How to interpret the ‘risk difference’ result?

Answer 12

Risk difference of 0 = both groups equally likely to have disease
Risk difference > 0 = First group more likely to have disease
Risk difference < 0 = second group more likely to have disease

Question 13

What is NNT?

Answer 13

NNT = Number needed to treat
Calculates effectiveness of an intervention/treatment
Number of people that need to receive the intervention before 1 person benefits from it

Question 14

How to calculate NNT?

Answer 14

1/(Risk difference in terms of proportion)

Question 15

What two tests could you use to compare a binary variable between two independent groups?

Answer 15

Chi-squared test
Or
Fisher’s exact test

Question 16

When would you use the Chi-squared test?

Answer 16

The total sample size is at least 40
OR
The sample size is at least 20 and there is an expected value of at least 5 in each cell

Question 17

When would you use Fisher’s exact test?

Answer 17

When the parametric assumptions for the chi-squared test do not hold

Question 18

How is the expected value for a cell calculated?

Answer 18

((Row total) X (Column total))/Total sample size

Question 19

What can proportion be used to infer?

Answer 19

The proportion tells you the risk of one person in a population having a disease

Question 20

If the odds of having a disease is 1.5, what does this mean about the population?

Answer 20

For every 1.5 people with the disease, 1 person does not have it