I have a set of data which I would like to find whether it is parametric or non-parametric in order to find out which test to do next to compare differences between 2 groups.

I have been told to do a histogram followed by Shapiro Wilk test which I have done.

The problem is that the data represents volumes in litres which can not be negative. I have 30 sets of values, so a fairly small number, and they range from around 0.80L to 4.0L. It is impossible for these volumes be below around 0.5/ above 5.0, even in the normal population, due to what they represent.

The histogram has shown that my data is not normally distributed because the smooth line has gone into the negatives. Furthermore, the Shaprio Wilk test has also shown my data is very far from being normally distributed, but again I feel it has probably taken negative values into account.

I am using SPSS.

Can anyone please give some advice?