I measured one dependent variable between three groups of data (control, group1, group2).The result of ttest indicates significant difference between control and group 1 (p vale =.04), control and group 2(p value .01), but not significant between group 2 and group 3.Do I still need to use the post hoc test? If so, which test is the most appropriate?

$\begingroup$ Kamran, I suspect you're at the wrong site for this question and are unlikely to get a good answer. We don't currently have many mathematicians here who are interested in statistics, but even if we did this question looks more like a practical question about scientific conventions rather than a theoretical question of interest to mathematicians. A similar question mathoverflow.net/questions/22532 did not get any answers. You and Pat could discuss whether there's a community for a statistics SE site, but I do not think MO is the place for that since we're mathematicians not scientists. $\endgroup$– Noah SnyderApr 26 '10 at 21:51

$\begingroup$ There is a proposal for a statistics SE site here: meta.stackexchange.com/questions/5547 $\endgroup$– Rob J HyndmanApr 29 '10 at 1:05
If you've got more than two groups, don't use ttests (due to the multiple comparisons trap). Use a oneway ANOVA instead (based on an Ftest). Then if you want a posthoc test after that, use Tukey's HSD.