Step 3: Identify data types and sources: Sample size and collection methods

Data must be drawn from a representative population with a wide variety of norms that could influence the results. For example, in studying the effects of fast food restaurants near schools, the study population of children with many fast food restaurants near their school should be balanced in terms of BMI, gender, socio-economic status and other factors that could influence the results. So must the control group. Of course, some research studies might wish to study intervention techniques in the context of specific population groups, like the effects of a diabetes management program upon a low-income population with a quite limited population base, demographically.

If an ideally balanced sampling is not possible to achieve, nonparametric statistics must be used to allow for an uneven breakdown of demographic data (Rumsey 2007, p.262). Chi-square tests create a balanced group from a large population sample, while the Fisher exact test allows the researcher to balance data of from small samples (Nonparametric statistics, 2010, Stat Soft). Nonparametric methods are useful when the researcher knows little of the parameters of the variable of interest in the population -- for example, if the socioeconomic status of the students might not be able to be known conclusively,...

"Simple tabulations count the occurrences of each variable independently of the other variables. Cross tabulations, also known as contingency tables or cross tabs, treats two or more variables simultaneously" (Nonparametric statistics, 2010, Stat Soft). Interpretation of the data will be shaped by the type of study -- for example, to compare two independent populations which are fairly equal, a signed rank test or t-test will be performed while the Wilcox signed-ranks test is more commonly used with irregular population samplings (Rumsey 2007, p. 295). The final step is to determine if the results are statistically significant enough to disprove the null hypothesis and prove the hypothesis of the researcher.
References

Marketing research. (2009). Quick MBA. Retrieved January 2010 at http://www.quickmba.com/marketing/research/

Nonparametric statistics. (2010). Stat Soft. Retrieved January 2010 at http://www.statsoft.com/textbook/nonparametric-statistics/

Rumsey, Deborah. (2007). Intermediate Statistics for Dummies. For Dummies.