The medical team of the cardiac surgery service of the University Hospital where we work is preparing a research protocol to present to the CEIm. They want to carry out a study to analyze the mortality of patients admitted to the hospital who have been vaccinated. By reviewing the medical records of admitted patients, they intend to verify that the length of stay in the hospital of vaccinated patients is 4 days less than in unvaccinated patients. In previous studies, a standard deviation of 5 days was demonstrated.

1. What minimum number of patients is necessary to include in the study in order to verify this unilateral hypothesis with a statistical power of 80% and a confidence level of 95%? Explain the process to obtain the sample size.

2. Bearing in mind that 20% of hospitalized patients do not know if they have been vaccinated or not, what number of medical records should you review?

3. How could it influence the result if the sample size is not reached?