Basic statistic math have two main calculation types: Time based and Sample based.

1.) Time based calculation takes values of signal at the specific time stamp. If the signal does not have a sample between two or more time stamps, the calculation takes the value of the previous sample until a new sample arises. 

2.) Sample based calculation takes only the the values of the signal samples. If the signal has a long period without any samples (asynchronous signal) , the calculation simply waits for the new sample.