Null Hypothesis Significance Testing

Hi All,

I am doing a question about NHST in my class, and I'm just stuck on one issue. I know what to do, but all of the data I'm working from is SPSS Output. I'm trying to decide whether or not to use the t or z distribution My questions are as follows:

1. I'm trying to do a two-tailed hypothesis that controls will be the same as the experimental group in an experiment. Is the standard deviation that SPSS Descriptives--Explore give you, the sample standard deviation? Or do I use either the standard deviation of the control or the experimental group.

2. When I'm doing a follow up 95% confidence interval of the difference between the two means... do I then use the standard deviationss of experimental and control group to calculate it?

Thanks so much!!!!