Implement different approaches to calculate the standard deviation for criterion 3

[milanwiedemann]

As raised during peer review, consider implementing different approaches to calculate the standard deviation for criterion 3. The method currently implemented in the package represents an independent sample t test.

• Consider the option to incorporate the correlation between the pre and post gain measures into this formula following Cohen (1988).
• Also consider the option to use only the standard deviation of the pre gain scores given that the standard deviation is maybe expected to change across the intervention Glass and colleagues (1981)

Cohen, J. (1988). Statistical Power Analysis for the Behavioral Sciences. New York, NY: Routledge Academic.
Glass, G. V., McGaw, B., and Smith, M. L. (1981). Meta-Analysis in Social Research. Beverly Hills, CA: Sage.