Thinking about populations and studies: Diabetes

Question 1

What is Prevalence?

Answer 1

• the frequency of ‘cases’ of a disease in a given population at a designated time
• prevalence = the number of people with the diseases/ number of people who could be at risk

Question 2

What is Incidence?

Answer 2

• the number of new cases of a condition in a given period of time expressed as a proportion of a population which is at risk
• could be a month or year, usually expressed as per 1000, 10,000, 1,000,000
• Incidence = no. new cases in a given time period/ total population at risk
  
  • ​what is defined as a new case needs to be clear

Question 3

What is the relationship between prevalence and incidence?

Answer 3

• prevalence depends on the incidence of disease and the time between onset and recovery (or death)
  
  • Prevalence = incidence x disease duration

Question 4

What can be gained from knowing the incidence of diabetes?

Answer 4

• Understanding diabetes and its risk factors (exposures) and outcomes
  
  • Accurate knowledge of the disease, trends, geographical differences → health care providers, researchers and policymakers
• Implications for
  
  • Individuals- health, happiness
  • Society – current and future economy, labour workforce
  • Informing prevention and public health interventions
  • Public awareness programmes + Educational programmes
• Service planning and commissioning
  
  • Screening and assessment
  • Staffing, training, resources, specialisms
• Identifying and prescribing targeted and indicated interventions: What, when, how, who
• Evaluating the effectiveness of interventions
  
  • Tracking changes in incidence before and after interventions

Question 5

What risk factors would be considered when looking at the likelihood of developing a disease?

Answer 5

• Demographic
• Behavioural
• Health-related
• Environmental
• Genetic

Question 6

How is the Incidence calculated?

Answer 6

Question 7

What is a cohort study?

Answer 7

• a group of people who have something in common
  
  • a group of individuals free from diseases or condition is selected usually at random
  • usually, there is an exposure of interest and participants are selected into exposed and non-exposed group
  • can be prospective or retrospective
• the main purpose is to identify the exposures (risk factors) that lead to a particular outcome
• in the end, they compare the disease incidence between the exposed and unexposed cohorts

Question 8

What is a Confounder?

• impact on cohort studies

Answer 8

• a variable that influences both the exposure and disease which causes spurious association
• a cohort study has more protection from a confounder as it establishes temporal precedence, however, there can still be this occurrence

Question 9

What are the strengths of a cohort study?

Answer 9

• more than one disease can be related to a single exposure
• can offer some evidence of a cause-effect relationship
• good when the exposure is rare
• minimises selection and information bias
• can calculate incidence rate and risk

Question 10

What are the weaknesses of a cohort study?

Answer 10

• Potential for losses to follow-up
  
  • attrition may differ in exposed vs unexposed
  • the longer the follow-up the greater the risk of attrition
• large samples usually required
• less suitable for rare diseases
• takes a long time to complete
• Expensive
• if a retrospective cohort study, the data availability and quality may be poor

Question 11

What sources can be used to gather cohort study data?

Answer 11

• Primary
  
  • survey data
• Secondary
  
  • hospital/medical records
  • mortality register
  • census data

Question 12

What are the pros and cons of using secondary data for a cohort study?

Answer 12

Pros

• cheaper, less time
• if anonymous, there is minimal ethical/ governance approval needed

Cons

• quality and availability may be limited
• poor accuracy and missing data

Question 13

What are the pros and cons of using primary data for a cohort study?

Answer 13

Pros

• accurate and specific data collected
• gather additional data

Cons

• lots of ethical and governance approval
• expensive + time consuming
• difficult to achieve representative sample

Question 14

What is the Relative Risk (or risk ratio)

Answer 14

• the risk of developing a disease in the exposed group compared to developing a disease in the unexposed group
• it tells us whether the disease is associated with exposure and the strength of the association

RR = incidence of disease among exposed/ incidence of disease among non-exposed

- [a/(a+b)]/[c/(c+d)]

Question 15

How can the relative risk be used in the analysis of a cohort study?

- RR < 1.0

- RR = 1.0

- RR > 1.0

Answer 15

• RR < 1.0
  
  • ​risk in the exposed group is less than the risk in the non-exposed group
  • the exposure may be protective against the disease
• RR = 1.0
  
  • ​risk in the exposed group is equal to the risk in the non-exposed group
  • the exposures is not associated with the disease
• RR > 1.0
  
  • ​risk in the exposed group is greater than the risk in the non-exposed group
  • the exposure may be a risk factor for the disease

Question 16

What does the RR number say about the strength of association?

Answer 16

• RR of 1.5 → risk of outcome 50% higher in the exposed than the unexposed group
• RR of 3.0 → risk in the exposed group is 3x as high as what is found in the unexposed
• RR of 0.8 → risk of outcome 20% lower in the exposed than in the unexposed group