Research Methodology 5: observational study designs Flashcards
What is prevalence ?
measures the frequency of “cases” in a given population at a designated time
Prevalence is expressed as a percentage
What is a cross-sectional study?
survey or test individuals in a population individually to determine the prevalence of disease
and exposure
The numerator (number of people with the diagnosis) is divided by the denominator (total population) to give the prevalence.
What is point prevalence vs period prevalance?
prevalence at a moment in time
However for things that fluctuate (e.g. hayfever)
period prevalence may be used e.g. hayfever symptoms over a summer
What are strengths of cross-sectional studies
Measure prevalence and thus disease burden in whole population and subpopulations
Can compare prevalence in exposed and non-exposed to risk factors
Quick and inexpensive
Can be used to initially explore a hypothesis, prior to another type of study
What are weakness of cross-sectional studies?
Not suitable for rare diseases
Not suitable for diseases of short duration
Cannot separate cause and effect as they are measured at the same time
Cannot measure rate of new cases arising and any changes therein
Describe cohort studies
A group of people is followed through time, and the onset of a disease/health event measured.
The incidence of the event/disease is compared among those exposed and those unexposed to a risk factor.
Define incidence
Incidence is the number of instances of illness/disease onset, in a given period in a defined population.
What are the strengths of the cohort study?
Can calculate incidence and relative risk
Can offer some evidence of cause – effect relationship i.e. impact of exposure on disease
Can identify more than one disease related to single exposure (and positive outcomes of exposure/s as relevant)
Good when exposure is rare
Minimises selection and information bias
What are the limitations of the cohort study?
Potential for losses to follow-up (attrition may differ in exposed versus unexposed / disease versus non, the longer the follow-up the greater the risk of attrition)
Often requires large sample, can take a long time to complete
Less suitable for rare diseases
Expensive
If retrospective, data availability and quality may be poor
What are case-control studies?
- retrospective
- clearly define two groups at the start: one with the outcome/disease and one without the outcome/disease.
- look back to assess whether there is a statistically significant difference in the rates of exposure to a defined risk factor between the groups
test association between exposure and outcome of interest
What are strengths of case-control studies?
Can offer some evidence of cause – effect relationship i.e. impact of exposure on disease
Can identify multiple exposures (both positive and negative associations, interactions)
Good when disease/outcome is rare
Minimises selection and information bias
Retrospective - cheaper and typically shorter in duration
What are weaknesses of the case control study?
Cannot calculate prevalence or incidence
Less suitable for rare exposures
Can be hard to ensure exposure occurred before onset
Retrospective data availability and quality may be poor
Suitable control group may be difficult to find
What is relative risk?
Incidence of disease in exposed divided by incidence of disease in unexposed
Odds ratio
Odds = outcome of interest / outcomes not of interest
i.e. what are the odds of stroke depending on exposure to exercise?
Odds ratio = odds in exposed / odds in non-exposed
What is the p value?
The p value is the probability that the difference observed could have occurred by chance if the groups compared were really alike.
