Statistics Module 1

In a randomised controlled trial of an intervention to stop smoking in patients with chronic obstructive pulmonary disease (COPD), the use of an intention-to-treat analysis is most likely to cause which one of the following?

A randomized, double-blind controlled trial compared the rate of major coronary events in diabetic patients treated with fenofibrate or placebo and followed for up to six years. The trial was negative on intention-to treat analysis (event rates 5.2% and 5.9% respectively, p=0.16). The figure shows dropout from and drop-in to other lipid-modifying treatment during the trial.

Assuming fenofibrate is efficacious, which of the following is most likely to explain the negative trial result?

Below are the tabulated results of five clinical trials for different drugs using death as the primary outcome. Follow-up time is five years for all trials.

The trial for which drug shows the lowest number needed to treat (NNT)?

The ‘number needed to treat’ (NNT) to prevent one event is a clinically meaningful measure of the efficacy of treatment. When comparing event rates between two treatment groups in a clinical trial, the NNT is calculated as:

In a clinical trial presenting two survival curves, the absolute benefit of treatment is best described by which of the following?

A new test may be useful as a screening test for Disease A. It is trialed with 100 patients attending a clinic, 10% of whom actually have Disease A, with the following results:

Based on these results, it is next trialed in a community population in which the prevalence of Disease A is 1%.

Which one of the following is most likely to fall?

To minimise the risk of human immunodeficiency virus (HIV)transmission by blood transfusion, which of the following is the mostimportant property of a blood screening test?

If the pre-test probability of a condition is known, which of the following is also needed to be able to
estimate the post-test probability?

The following table shows the results of a study of a new diagnostic screening test for colon cancer performed in a tertiary referral hospital on patients referred with rectal bleeding.

When the same test is applied to a group of asymptomatic patients in general practice, which of the following will be most likely to increase?

The following table shows the results of a study of a new diagnostic screening test for multiple sclerosis

In this study, which of the following measurements of test performance has the highest calculated

A new D-dimer assay has a sensitivity for deep venous thrombosis (DVT) of 95% and a specificity of 50%. It is proposed to use it to screen a group of passengers after long distance travel. Previous data have suggested this group has a pre-test probability of DVT of 1%.

What is the best estimate of the post-test probability of DVT in an individual with a positive D-dimer result?

A test has a sensitivity of 95% and a specificity of 90%. It is used to screen the general population for a rare condition that has a prevalence of 1 in 100,000.

What will the positive predictive value be nearest to?

A test has a sensitivity of 95% and a specificity of 80%. It is used to screen for a condition with a prevalence of 1 in 100.

What will the positive predictive value be nearest to?

A certain autosomal recessive disorder affects 1 in 1600 people; the carrier frequency is 5%. A DNA assay can identify the mutation in 80% of carriers; the false-positive rate of this assay is zero.

What is the best estimate of the positive predictive value (PPV) and negative predictive value (NPV) of this assay in screening the population for carriers?

You admit an 40 year old man to the Intensive Care Unit with septic shock. A randomised controlled trial recently showed that a new drug reduces mortality. In the study, 33 of 110 control patients (30%) died, compared to 30 of 120 (25%) in the treatment group.

It costs $2,000 per treatment course.

In order to save one life using this new drug, about how much would your hospital have to spend?

The odds ratio provides a reasonable estimate of the relative risk for uncommon conditions.

In calculating the odds ratio, what does an odds ratio of 0.6 mean?

A disease has an annual incidence of 15 cases per 100,000. The mean survival after diagnosis is five years.

What is the best estimate of the prevalence of this disorder?

Your region has a birth rate of about 50,000 babies per year. About 50 preschool children are diagnosed each year with “crumblitis”, a rare gastrointestinal disorder, the cause of which is unknown. You hypothesise that this condition is related to maternal ingestion of orange rhubarb during pregnancy.

Which one of the following is the most appropriate research design to test your hypothesis?

Which of the following clinical study design is NOT considered an observational study?

A new highly sensitive D-dimer assay has a sensitivity of 99.5% but a specificity of only 50%.

What is the likelihood ratio for a positive result nearest to?

The curve that graphically represents the family of cutoff points for a positive vs. negative test is a receiver operating characteristic (ROC) curve. The area under this curve is a quantitative measure of the information content of a test. The ROC axes are

A rapid finger-prick blood test to help diagnosis deep vein thrombosis is developed. Comparing the test to current standard techniques a study is done on 1,000 patients:

What is the specificity of the new test?

As part of a research project you are trying to ascertain whether the use of dummies in infants is linked to sudden infant death syndrome.

What is the most appropriate form of study design?

You are reviewing a new study on the benefit of omega-3 fish oils in patients with established ischaemic heart disease. What does the power of the study equate to?

A new test to screen for pulmonary embolism (PE) is used in 100 patients who present to the Emergency Department.

The test is positive in 30 of the 40 patients who are proven to have a PE. Of the remaining 60 patients, only 5 have a positive test.

What is the sensitivity of the new test?

Which one of the following steps in the design of a clinical trial of a new therapeutic agent most reduces the chance of a type II error?

A researcher finds that people who have strokes are generally taller than people who do not have strokes.

Which of the following statistical tests would be most appropriate to determine the statistical significance of this finding?

The following results were obtained from a randomized controlled trial of a new artificial surfactant called “S”.

The number of babies that you would need to treat with S to prevent one case of chronic lung disease is:

A double blinded randomized control trial for a new drug for prevention of Alzheimer’s disease in a group of at risk patients was conducted over a 10 year follow up period.The absolute risk of developing Alzheimer’s disease in the group receiving placebo was 10%.The relative risk of those with the new medication was 0.8.

What number of patients will need to be treated with active drug for 10 years to prevent one new case of Alzheimer’s disease?