Epidemiology basics

Question 1

what are the properties of a good screening test?

Answer 1

Reliable: consistent results from different observers on different occasions

Practical: the test is easy to perform and interpret

Acceptable: people are happy to have the test performed on them

Sensitive and specific: correctly identifies those with the disease and excludes those who don’t have the disease

the condition is an important health problem and detection of it will change management

there should be a recognisable latent and symptomatic stage

there is available treatment for the condition

test should be cost effective

Question 2

what do we mean by positive predictive value?

Answer 2

Positive predictive value is the probability that subjects with a positive screening test truly have the disease.

Question 3

what do mean by a negative predictive value?

Answer 3

Negative predictive value is the probability that subjects with a negative screening test truly don’t have the disease

Question 4

what do we mean by odds ratio? what is the pre-test odds and probability if the prevalence of depression in a certain population was 5%?

Answer 4

Odds is the ratio of events to non-events.

A prevalence of 5% means that for every patient who is depressed, there are 19non-depressed patients.

Hence, the pre-test odds is 1in 19.
The pre-test probability is 1in 20.

Question 5

what happens to specificity and sensitivity and positive predictive value and negative predictive value with changes in the prevalence of a disease x in a population?

Answer 5

Sensitivity and specificity are characteristics of a diagnostic test and do not change with prevalence of the population.

Positive predictive value increases with the prevalence, while negative predictive value decreases with the test.

Question 6

what is the advantage of randomisation in a study?

Answer 6

reduces selection bias

Question 7

What is an “intention-to-treat analysis?”

Answer 7

This means that during the analysis of the study results, patients remain in the groups to which they were randomized in the beginning of the study, even if they are unable or unwilling to complete the treatment.

Question 8

what is the most important factor that may improve the power of the study?

Answer 8

increasing sample size