Measurement Scales , Nimbers , Ratios

Question 1

We want our sample population to be :

Answer 1

1) representative
2) unbiased
3) precise

Question 2

What are the two types of errors that can occur in a study that may influence the results of a study ?

Answer 2

1) RANDOM ERROR ( chance ).

2) SYSTEMATIC ERROR( bias)

Question 3

What is a random error ( chance) ?

Answer 3

• this is caused by sampling variations

- as the sample size increases however , the random error would reduce.

Question 4

What is a systematic error ? ( bias)

Answer 4

• difference between the true value and the expected value

- bias does not reduce when the sample size increases - it remains the same

Question 5

With random errors , as sample size increases precision increases/decreases?

Answer 5

Increases as uncertainty is reduced

Question 6

What are the two types of biases?

Answer 6

1) selection bias

2 information bias

Question 7

What are 3 examples of selection bias ?

Answer 7

1) study sample ( external validity)
2) group selection within a study ( internal validity )
3) healthy worker effect

Question 8

What is study sample selection bias

Answer 8

• also referred to as external validity

- this is where the study sample is to a representative of the entire population of interest

Question 9

What is group selection bias within a study ?

Answer 9

• groups within a study may not be comparable
• this is often referred to as internal validity
• for example old people and young people within a sample.

Question 10

What is the healthy worker effect?

Answer 10

Workers usually exhibit lower overall mortality than the general population

Question 11

What are 4 examples of information biases ?

Answer 11

1) recall errror
2) observer or interviewer error
3) measurement error
4) misclassification

Question 12

What is recall error ?

Answer 12

Differences in recollection from study participants regarding past events or experiences

Question 13

What is observer / interviewer error ?

Answer 13

Study observer or interviewer may have preconceived expectations or knowledge that may influence the result

Question 14

Measurement error

Answer 14

Differences in the measurement of participants

Eg using the same tool to measure something you may obtain different results from the same person each time

Question 15

What is misclassification error

Answer 15

This is when we classify participants into the wrong group

For example putting a patient in a diseased group when they are not diseased

• this usually arises from a measurement or observational error

Question 16

Measurements in epidemiology: define prevalence

Answer 16

The proportion of people who have a disease at a given point in time

• number of people with disease ( both old and new cases) / total population ( number of people)

Question 17

Measurements in epidemiology: define incidence

Answer 17

The number of new cases of a disease within a given time frame

• focuses on NEW cases only
• useful when monitoring epidemics
• often reported as a ‘ rate ‘ eg 50 Per 100,000 person years

Incidence rate = number of new cases for the disease/ 3 patient time at risk

Question 18

What is the incidence rate ratio

Answer 18

Compares the incidence rate in one group to incidence rate in another group

IRR = incidence rate in group A / incidence rate in group B

Question 19

Is odds a ratio ?

Answer 19

Yes

Question 20

Calculate,ate the odds of disease for group A and B

Answer 20

Group A= ( disease) a / non disease (b)

Group b = disease (c) / non disease (d)

odds ratio is a ratio of ratios. : odds of group A ( a/b) / odds of group B (c/d) = ad /bc

Question 21

Is risk a ratio ?

Answer 21

No it is a proportion

Question 22

Calculate the absolute risk for group a and b

Answer 22

Group A : a/a+b

Group b : c/c+d

Question 23

If the value for. Odds ratio is lower than 1 what does this mean ?

Answer 23

The group represented in the numerator has a lower odds of the event

Question 24

If the is less than one , what does this mean ?

Answer 24

The group represented in the numerator has a lower risk of the event

Question 25

If the odds ratio is greater than 1, what does this mean ?

Answer 25

The group represented in numerator has a greater odds of the event

Question 26

If the RR is greater than 1 , what does this mean ?

Answer 26

The group represented in the numerator has a greater risk of the event

Question 27

When would the RR/OR being less than 1 be a desired outcome ?

Answer 27

When the event is a bad outcome

Question 28

When would the OR / RR being greater than 1 be a desired outcome ?

Answer 28

This is diesired if the event is a good outcome

Question 29

What is a confounding variable ?

Answer 29

A variable is an extra variable that you didn’t account for - they can ruin experiments by affecting the dependant variable

Question 30

How should we tackle the effects of confounding variables ?

Answer 30

1) standardisation -

However a lot of confounding variables that are unknown and are very difficult to addrsss

Question 31

Define relative risk or risk ratio

Answer 31

Describes the comparison of the probability of an event occurring in one group compared to another

Question 32

Define risk difference

Answer 32

Describes the absolute difference in risk between one group compared to another