STUDENTS TEST AND THE GLASGOW COMA SCALE

Study objective: To explore how Student's t test, which assumes normal errors, is affected by the nonnormal distribution of Glasgow Coma Sca le (GCS) scores.Methods: A sample of 145,295 GCS scores from the Penns ylvania Trauma Systems Foundation was assumed to represent the true GC S distribution. Four Monte Carlo simulations, each based on 40,000 rep lications, were conducted to approximate the distribution of t values arising from samples drawn from the GCS distribution, using sample siz es of 10, 30, 60, and 100 per group. Results: The histograms of t valu es derived from the GCS distribution resembled the corresponding Stude nt t distributions for sample sizes of 30 or more per group. The t tes t was conservative in samples of size 10, conservative to acceptable i n samples of sizes 30 and 60, and acceptable in samples of size 100. I n no case was the t test liberal. Conclusion: The t test can be applie d to GCS data with a type I error rate that never exceeds the investig ator's chosen significance level. However, a sample size exceeding 30 per group is required for the error rate to adequately approximate the chosen significance level.