If you make a stem-and-leaf plot of medical student ages, you will probably have to split it to see the distribution, as the range is very small. These authors measured medical student age in their article, and report mean and standard deviation: http://www.journalofsurgicalresearch.com/article/S0022-4804(06)00525-7/abstract
Linear mixed-effects models using R : a step-by-step approach / Andrzej Galecki, Tomasz Burzykowski. 2013. Máis información: http://www.springer.com/statistics/statistical+theory+and+methods/book/978-1-4614-3899-1
A statistical model consists of some assumptions that are observed after insights are produced from big data. The insights are then used to form a model so that you can categorize them according to their diversity. This infographic describes exactly which statistical model you should choose so that it meets your need.