Most dissertations require you to conduct a power analysis. That is, given a power of .80, how many participants will you need for your study? Power is simply the probability of not making a type II error.
To conduct a power analysis, the number of participants needed depend on several factors. These factors include the power for your study (typically .80), the effect size, and the alpha (typically .05). Effect sizes (small, medium, and large) tell you the strength or importance of a particular relationship, and are unique to the type of statistical analysis that you want to conduct.
For any type of analysis—regression, ANOVA, chi-square, t-test, structural equation modeling, time-series, cluster analysis—we can conduct a customized one. For a select group of analysis, it can be conducted and written up with references using our online sample size/power analysis tool.