public health equations

Question 1

What is point prevalence?

Answer 1

number of cases of disease at a point in time / total number of people in the defined population at the same point in time

Question 2

what is period prevalence?

Answer 2

Period prevalence is the number of individuals identified as cases during a specified period of time, divided by the total number of people in that population.

Question 3

What can increase prevelence?

Answer 3

screening programmes identifying new cases

increasing risk factors

increased life-expectancy due to better treatments can increase prevalence

Question 4

What is incidence?

Answer 4

The number of new cases per unit time (can be expressed as a percentage or per e.g. 100,000). e.g. 100,000 new cases per year

Question 5

What is incidence rate?

Answer 5

Number of persons who have become cases in a given time period / total person-time at risk during that period eg 3/32 = 9.4 per 100 person years

Question 6

What is absolute risk?

Answer 6

the incidence divided by the population.

Gives a feel for the actual numbers involved i.e. has units (e.g. 50 deaths/ 1000 population)

Question 7

What is attributable risk?

Answer 7

The rate of disease in the exposed that may be attributed to the exposure

Attributable risk = incidence in exposed – incidence in unexposed

It’s about the size of the effect in absolute terms – gives a feel for the public health impact if causality is assumed

The attributable risk:
AR = (A/(A+B)) – (C/(C+D))

The attributable risk percentage of smoking can be calculated as:
AR % = AR / (A/(A+B)) x 100

This means 53.31% of incidence of cardiovascular disease among smokers is attributable to their smoking.

Question 8

What is relative risk?

Answer 8

Ratio of risk of disease in the exposed to the risk in the unexposed

Relative risk = incidence in exposed / incidence in unexposed

Tells us about the strength of association between a risk factor and a disease

Relative risk of 1.74 = 74% more likely

An RR of 1.00 means that the risk of the event is identical in the exposed and control samples. An RR that is less than 1.00 means that the risk is lower in the exposed sample. An RR that is greater than 1.00 means that the risk is increased in the exposed sample.

Question 9

What is relative risk reduction?

Answer 9

The reduction in rate of the outcome in the intervention group relative to the control group

1 minus RR

(incidence in non exposed – incidence in exposed) / incidence in non-exposed

Question 10

What is absolute risk reduction?

Answer 10

The absolute difference in the rates of events between the 2 groups

Gives an indication of the baseline risk and the intervention effect

Incidence in nonexposed – incidence in exposed

i.e. assuming exposed means they have had a particular intervention (such as giving statins to people with hypercholesterolaemia and then a control group who do not have statins and seeing how many in each group have a heart attack to see if the intervention of statins is effective

Question 11

What is number needed to treat?

Answer 11

the number of patients we need to treat to prevent one bad outcome

NNT = 1/(risk in non-exposed – risk in exposed)

Aka 1/absolute risk reduction

Question 12

When is odds ratio used?

Answer 12

For case control studies it is not possible to calculate the relative risk and so the odds ratio is used.

For cross-sectional and cohort studies both can be derived but odds ratio is used if it is not clear which is the IV and which is the DV because it is symmetrical.

Question 13

What is odds?

Answer 13

The odds of an event is the ratio of the probability of an occurrence compared to the probability of a non-occurrence.

Odds = probability/(1-probability)

eg probability = 0.75

Odds = 0.75/(1-0.75)
Odds = 3

Question 14

Interpretation of odds ratio?

Answer 14

OR=1 Exposure does not affect odds of outcome

OR>1 Exposure associated with higher odds of outcome

OR<1 Exposure associated with lower odds of outcome

Question 15

What is cumulative incidence/risk

Answer 15

number of new cases in a time period/ number of disease free people at the start of the time period.

Cohort study would do this

Denominator is disease-free people

It is a proportion.
Time period must be stated.
Closed population/cohort

Question 16

What is incidence odds/odds of disease?

Answer 16

number of new cases of disease in time period/ number of people who were still disease free at the end of the time period so is probability of disease/probability of not getting disease.

Eg 25/75 if 25 out of 100 get disease. Odds of disease

Question 17

calculate attributable risk and attributable risk percentage :
disease no disease
smoked 25 140

non-smoked 52 683

Answer 17

The attributable risk of smoking can be calculated as:
AR = (A/(A+B)) – (C/(C+D))
AR = (25/(25+140)) – (52/(52+683))
AR = .08077

The attributable risk percentage of smoking can be calculated as:
AR % = AR / (A/(A+B)) * 100
AR % = .08077 / (25/(25+140)) * 100
AR % = 53.31%
This means 53.31% of incidence of cardiovascular disease among smokers is attributable to their smoking.

Question 18

What is odds ratio?

Answer 18

The odds ratio is the ratio of odds for exposed group to the odds for the not exposed groups.

Odds ratio = (AxD) / (BxC)

value > 1 is positive

Question 19

define confidence interval

Answer 19

A range of values in which we are 95% certain that the true value lies

Question 20

define statistical power

Answer 20

Statistical power is the probability of detecting an effect if there is a difference present.

More simply it means that if there was a difference in a particular factor between two groups, how likely are we to find this difference?

Power can be increased by using a greater sample size, using more precise measuring instruments, and using a higher significance value.