• Observational studies include: cohort; case-control; and cross sectional studies
• They are non-randomised & non-experimental

• Cohort study
  • Prospective 
  • Used to determine aetiology & natural history of disease
  • Define cohort –> assess risk factors to follow –> follow forward in time
  • Used to calculate risk ratio
  • Risk ratio indicates increased/decreased risk of disease associated with factor of interest

Advantages	Disadvantages
Time sequence can be reviewed	Costly
Provides info on wide range of disease outcomes	If outcome is rare – large sample sizes are required
Incidence/risk of disease can be measured	Long follow up time – increased patient drop out
Can gather info on range of risk factors	Disease outcomes/aetiology may change over time
Can be used to study rare exposure
Changes over time can be studies
Reduced recall bias & selection bias

• Case control study
  • Retrospective
  • Review cases & controls to establish risk factors
  • Cases & controls can be matched to use participants who are similar in each group
  • Good for rare diseases
  • Used to calculate odds ratio 
  • Odds ratio indicates the odds of disease in individuals exposed and unexposed to a factor

Advantages	Disadvantages
Quick, cheap, easy to perform	Increased recall bias
Good for rare diseases	Not suitable for rare exposures
Range of risk factors can be studied
No loss to follow up

Risk ratio & odd ratio are similar when prevalence of disease is rare

• Cross sectional
  • Observe subject group once & review relationships between observed variables –> prevalence

Summary measures of disease

• Cancer registration
  • Continuous & systematic collection of data on all cancer patients in a defined population
  • Held by Public Health England
  • Cancer registry will analyse, interpret & report the data periodically
  • Collected from: death certificates; hospitals; healthcare facilities/labs in the area
  • Data collected inc: ID; baselines characteristics; type & stage of cancer; treatment received; progress; survival; history
  • Used for: healthcare planning & monitoring & aiding research

Screening Tests

• Sensitivity – ability to detect disease correctly in those with it
• Specificity – ability to identify no disease correctly in those who do not have it

• Sensitivity & specificity are constant regardless of how prevalent the disease it

• Importance of each may depends on the context of the test….
  • Screening tests in primary care – prioritise sensitivity – ability to correctly identify disease
  • Diagnostic test in secondary care – prioritise specificity – ability to exclude those without disease

• Positive predictive value – proportion of test positive results that are true positives
• Negative predictive value – proportion of test negative results in those that are true negatives

• PPV & NPV will depend on tests ability to detect disease & underlying population
  • Increased prevalence – increased PPV
  • Increased prevalence – decreased NPV