When designing a study, you can't have perfect internal and external validity at once, so choose what matters most for your research question. Control what you can through design, pick the right statistical tests, and accept the tradeoffs.
The next thing I want to do is understand the interplay between internal and external validity. No study is going to be perfect, so we control for as much as we can in design. You need a design that is going to answer your questions. You need a design that you can justify that will answer your questions. You need in a quantitative study, statistical analysis that's appropriate. Drop the word help in the comments and I'll send you a link to my free dissertation help community.
