All, T-Test & Z-Test are used to check means of two groups. T-test is used when groups are small & variance is not know otherwise Z-test is used to check regarding mean diff. in two groups. F-Test & Chi-Square are used to check variance of two groups. When each of these is used. I am not a statistics student but am a professional in OLAP reporting; am trying to learn Data Mining(Stats) on my own. Your help in removing my stat concepts confusions can save a lot of time for me. Thanks. Judge