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A new test to diagnose UTIs is being evaluated. The sensitivity of the test is 70% and the specificity is 90%. In the study there are 100 patients who actually have UTIs and 200 who actually do not, as determined by the diagnostic gold standard. How many false positives are in this study?
- Specificity = TN / (TN+FP)
- .90= TN/ 200
- TN = 180
- TN + FP = 200
- 180 + FP = 200
- FP = 20
A large cohort study is conducted to assess the association between smoking and squamous cell carcinoma of the esophagus among middle-aged Chinese men. During 10 years of follow-up, smokers have 5 times the risk of esophageal carcinoma compared to non smokers (relative risk = 5.0, 95% confidence interval = 2.9-7.1). According to the study results, what percentage of squamous cell carcinoma of the esophagus in smokers can be attributed to smoking?
- AR= [(RR - 1)/ RR]
- AR= [(5-1)/ 5]
- AR= 0.8
- AR= 80%
- Therefore 80% of esophagus squamous cell carcinoma cases in smokers were attributable to smoking.
A study determines that the mean blood cholesterol level is 195 mg/dL in 200 non-diabetic hospitalized patients and 210 mg/dL in 180 diabetic hospitalized patients. The probability that the observed difference is due to chance alone is reported to be 5%. There is also a 20% probability of concluding that there is no difference in blood cholesterol level when there is one in reality. What is the power of the study?
- The power of a study is the ability to detect a difference between groups when such a difference truly exists.
- Power = 1 - β
- The probability of concluding that there is no difference in blood cholesterol level when in reality there is one is 20%
- Power= 1-.20
- Power =0.80
What is the Hawthorne/ observer effect?
The tendency of study subjects to change their behavior as a result of their awareness that they are being studied.
The tendency of study subjects to change their behavior as a result of their awareness that they are being studied is known as?
Hawthorne effect (observer effect)
A new estrogen receptor agonist is being evaluated for the treatment of postmenopausal symptoms. A prospective study shows that the drug increases the risk of DVT in treated women who, compared to untreated women who smoke, with a relative risk (RR) of 1.70 and p-value of 0.01. In nonsmokers, no increased risk of DVT is evident with use of the drug (RR- 0.96; p-value=0.68). What is this phenomenon?
- Effect modification.
- Note* the p-value of 0.68 is statistically insignificant.
- Effect modification occurs when the effect of an exposure on an outcome is modified by another variable.
- In this scenario, smoking status modified the effect of the new estrogen receptor agonist (exposure) on DVT incidence (outcome).
A study evaluated the role of ACE inhibitors in the prevention of coronary events in patients with diabetes. During 5 years of follow-up, 120 out of 400 diabetic patients who had taken an ACE inhibitor developed an acute coronary event. Over the same time, 100 out of 30 diabetic patients who had not taken an ACE inhibitor experienced coronary events. What was the relative risk of developing a coronary event in diabetic patients who were taking ACE inhibitors compared to diabetic patients who were not?
- RR= [a/(b+b)] / [c/(c+d)]
- RR = (120/400) / (100/300)
- RR= 0.90
An experimental drug X is being tested for the treatment of stage IV solid tumor malignancies. Part of the drug company's evaluation process is to analyze survival data after 3 months of treatment. The results are given in the table below.
What is the number needed to harm for drug X?
- NNH= 1/AR
- Attributable risk = Event rate treatment - Event rate placebo
- Adverse event rate in treatment group= 60/80 = 0.75 = 75%
- Adverse event rate in placebo group = 38/ 76 = 0.50 = 50%
- NNH= 1/ (0.75-0.50)
- NNH= 1/ 0.25
- NNH= 4
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