Clinical Trials

Clinical Trials: A Practical Guide ■❚❙❘Table 2. Summary of systolic blood pressure (SBP) (mm Hg) by treatment and visit in a drug trial.Statistics Visit 1 (Baseline) Visit 2 (2 weeks) Visit 3 (4 weeks) Visit 4 (6 weeks)Drug Placebo Drug Placebo Drug Placebo Drug PlaceboNo. of patients 1,013 1,015 1,001 994 978 969 975 958No. of unknowns 0 0 12 21 35 46 38 57Mean 129.94 130.32 124.54 129.46 123.19 129.72 121.42 127.90Standard deviation 18.97 18.52 20.05 18.51 20.48 19.34 20.60 18.7125th percentile 117 118 110 116 110 118 108 114Median 130 130 122 130 120 130 120 12875th percentile 140 140 140 140 138 141 134 140There are three measures commonly used to summarize the spread of a variable:• standard deviation: this gives an indication of the average distanceof all observations from the mean. The standard deviation has animportant role in statistical analysis• range: the difference between the lowest and highest valuesis known as the full range of values• range between percentiles: percentiles are the value below whicha given percentage of the data observations occur. A common rangeused is the interquartile range, which is the range between the 25thand 75th percentile. Using this overcomes the problem of extremedata values away from the mean or medianEach measure has its own advantages, but the standard deviation is morecommonly used and is more often applied in statistical inference. For survivaldata, the median and range are often used to describe the central locationand spread.ExampleIn the clinical report of a pharmaceutical trial, descriptive statistics are oftenprovided, such as number of observations, mean, standard deviation, median, and25th and 75th percentiles by treatment. Table 2 is such a table extracted from aclinical report, summarizing the SBP change at different visits in a randomizedplacebo-controlled clinical study assessing the effect of a study drug on reducingSBP. The table was designed to provide a quick reference to summary measuresacross the treatment groups at each visit. Three types of summary measuresare tabulated.173