How to calculate significant differences in correlation using stata/spss


This is probably a fairly simple question but I don't know to handle it properly.

I have monthly data of 2 categories. Each category contains 8 assets, and of each asset I have the monthly returns for the period 1995-2014. I calculated the correlation matrix for each of the two categories and I have also computed the correlation matrix of the the categories combined using STATA.

In my hypothesis, i expect that the 8 assets of category 1 are more correlated with each other in the 2008-2010 period than they are in the other 17 years. How can I calculate this and draw statistically significant conclusions using STATA?

In another hypothesis, i expect that the average returns of category 2 are higher in the 2008-2010 period compared to the average returns of category 1.

Finally, i expect the variance of returns in category 2 are lower in the 2008-2010 period compared to category 1.

So how to calculate this? I can calculate the returns and variance of each individual category or asset and then compare the results with another category/asset but i Don't know how to draw significant conclusions from that using stata/spss.

Sorry for my english, as I am not a native english speaker.

Any help is much appreciated!

Also, I expect that the average
To compare correlations between disjoint time periods or asset groups you can use an independent samples t-test (look up "t-test for correlation" and extend it). Alternatively, in almost any situation, you can use bootstrap.