How to calculate p value

The more data you have, the smaller a difference will be considered "statistically significant". Whether 25 is enough depends on how big a difference you are trying to detect.

The first thing you need to do is define your independent variables. One of them is "appropriate mattress, YES or NO". Are there other variables that are known to be significant? Body Weight? Gender? Age? Length of hospitalization? Any variable that is already known or suspected of contributing to pressure ulcers should be recorded. I found this reference.

More to your question, Excel has a few functions you can use depending on the number of independent variables in your data. If you us 1 and 0 for your appropriate mattress question, and you don't have any other independent variables, you could use TTEST. If you have several independent variables and they are fairly linear, you could use LINEST.

Many thanks for your answer!