Mortality Risk Factors in Head Injury ICU

As there was no similar previous study conducted using the time to an event data model, the sample size was determined by using single population proportion formula with the following assumptions; 52.8% proportion of death among head injury patients in the ICU [17 ], 95% confidence level and 5% margin of error. Based on this, the actual sample size for the study was computed by using the following formula: $$\mathbf{n}=\frac{\left(Z\right)2\text{P}\left(1-\text{p}\right)}{{\text{d}}^2}=\frac{\left(1.96\right)2\left(0.528\right)\left(1-0.528\right)}{\left(0.05\right)2}=\mathbf{383},$$ where n = sample size, Z = critical value of 95% CI = 1.96, P = Proportion mortality among trauma patients in the ICU (52.8%) = 0.528, D = precision (marginal error) = 0.05.
Thus, by adding 10% for possible incomplete charts and lost medical records, the total sample size was 421.