All the variables are not the same! The number of days of hospitalization and the type of operation are slightly different. And that difference is important when it comes to statistics ...
In EasyMedStat, there are 4 types of variables: list, yes-no, numeric and event.
You use it when you have a limited number of options for the answer.
For example, the type of operation could be for your series on the knee surgery one of: Meniscectomy, Total arthroplasty, Unicompartmental arthroplasty, ACL Ligamentoplasty, Osteotomy.
So you have 5 possible answers for the question: "What operation was performed on the patient ?". This is a "List" variable. In statistics, it is also called a discrete multinomial variable.
You use it when the answer to the question can only be yes or no. For example, Smoker: yes or no? Epilepsia: yes or no? This type of variable should be preferred over List variables whenever possible because it allows more statistical test power.
Use it when the data is a number which can take any value (the number of possible values is not known before the study).
For example, the weight of the patients could go from 40.0 kgs to 110.0 kgs but with a lot of intermediary values: 67.3, 81.2, ...
In this case, it is better to use the Numeric type. In statistics, it is also called a continuous variable.
You use this type when you want to track an event that could happen to a patient.
For example, the re-intervention date or the date of a complication could be a noticeable event you would like to track. This is the only type of variable that allows making survival analysis (Kaplan-Meier, …)