Finding Median of Dataset or create function to Datawatch

Hi All

I have the following dataset that I have managed to Model the data which is as I wanted.

I required from the "Response" values to calculate the "median". I know Modeller can do all sorts of functions, but could not find this one. I assume that modeller has not got this because the system has to order the dataset and pluck out the middle value. Or it is there and I am not seeing the correct function? This maybe a silly question but can you create your own functions, somewhere in the options of "Click to Add new function" or "manage external functions..."

Maybe someone can give me some advise on best doing this. Here is my data set. It is very straightforward in terms of modelling.

All advise welcome.

Thanks

Ravi