In a study comparing drug A and a placebo control, 20 out of 200 patients taking drug A die after 3 years. Twenty-five out of 225 patients taking the placebo die after 3 years.
If death is the outcome of interest, the control event rate is given by:
A. Drawing a 2 × 2 table will help answering this question
Control event rate is the rate of death (‘event’ of interest) in the control group = 25/225
Reference:
In an RCT comparing the effect of exposure therapy versus cognitive restructuring, follow-up was carried out at 6, 11, 24, and 36 weeks. At weeks 6 and 11, after rating the patient, the outcome assessors tried to guess the treatment condition. Correctness of guesses did not differ significantly from that expected by chance.
This was an attempt to demonstrate which of the following?
C. Adequacy of blinding can be tested during or after completing a trial by asking the blinded parties to guess the allocation. Guess rates that are significantly higher than expected by chance indicate failure of blinding. Testing for ‘blindness’ may not generate valid answers all the time. This is because as participants begin to experience treatment response or outcomes of interest, they begin to generate ‘hunches’ about the efficacy of the treatments being tested. Hence tests for blinding can show spurious failure of blinding while in fact they test the ‘efficacy hunches’ that develop late in the process of a trial.
If the sample size is sufficiently large, mean values of repeated observations follow normal distribution irrespective of the distribution of original data in the population.
This is known as:
B. The central limit theorem explains why normal distributions are so frequent when considering most biological parameters. Consider repeated sampling from a population where distribution of the observed variable is unknown. You intend to plot the distribution of individual means of each sample from the population. As sample size increases, the sample means approach a normal distribution with its mean value being the same as the population mean and a standard deviation equal to the standard deviation of the population divided by the square root of the sample size. Usually 10 or more observations are sufficient to result in an approximate normal distribution.
The validity of a new instrument is compared with an external criterion. A conceptually related external criterion is identified to occur sometime in the future. If the correlation between current scores obtained using the instrument and the future expected outcome is studied, this is called:
C. The term validity refers to the strength of our conclusions, or in the case of statistics, the strength of our inferences. It refers to applicability. The term reliability refers to the consistency of our measurements, or the reproducibility. An important subtype of validity is called criterion validity. If an instrument provides a result that withstands the test of an external criterion then the instrument is said to have high criterion validity. The external criterion may be a measurement that can be obtained more or less at the same time (concurrent validity) or it may be an outcome that is predicted to occur in the future (predictive validity). If a test offers something over and above what is offered by an existing instrument, then incremental validity can be established. Internal consistency of a test refers to looking at how consistent the results are for different items (measuring the same construct) within the instrument studied. This can be measured by undertaking item–item correlation, item–total score correlation or split half reliability (Cronbach’s alpha; see elsewhere in this chapter).
A recent study conducted in a palliative care unit assessed the use of a two-item questionnaire to screen for the presence of depression. Given below is the table which compares the result of the screen to the gold standard (DSM-IV) diagnosis. In relation to this table, answer questions 70–82
The sensitivity of the overall screen using both items is approximately:
E. Questions similar to this are very common in the MRCPsych exam. Most of such questions provide some data and require the candidate to do a series of calculations from the data. It is always advisable to redraw as soon as possible the presented data in a format that will fi t the purpose. From the given table we can create a 2 × 2 table, with the gold standard result on the top. One should be careful while constructing the 2 × 2 table. It is advisable to stick to one style of using columns and rows to indicate a particular group of data. Here, we have drawn the 2 × 2 table with the gold standard results indicated across the two data columns with screening test results indicated across the two rows.
Sensitivity is defined as the test’s ability to identify people who, according to the diagnostic (gold) standard, actually have the disorder (true positives). Sensitivity = A/(A + C) = 39/43 = 90.69%, i.e. 90.69% of subjects who really have depression according to DSM-IV criteria have a positive test result on the screening test. In other words, sensitivity is the proportion of true positives (cases) correctly identified by the test.
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