Testing the significance of change between two groups

Hi all,

I was hoping for some advice on how to test the change between two groups.

First of all, my groups are relatively large (n=100-150). For each member of the two groups I have the following: I have observed 'x' many of something in group one, and have extrapolated this to expect 'y' many in group two. I have observed 'z' in group two, and so would like to test the significance between 'y' and 'z'.

Would a chi-squared test be appropriate for this? Or would I potentially find effect through chance?

Alternatively, would it be more appropriate to average 'y' and 'z' and use an independent groups t-test?

Any guidance would be much appreciated.

Many thanks in advance,