Control groups get no intervention, experimental groups do. The point is isolating whether your intervention actually caused the effect. A study of 100 people over weeks shows the problem: if a major event happens mid-study, it can bias the last 60 responses and wreck your results.
In quantitative studies, we've got a control group, meaning that's a group that's not getting any kind of intervention of whatever you're studying. And then we've got an experimental group, and they get the intervention. And that means hopefully we've controlled for other confounding factors that can happen even then. Let's say I'm doing a study over a couple of weeks time. I'm collecting data from 100 people, and I get 40. And then there's a major incident that happens in the world, or a major announcement that happens that somehow applies to my study, and then I collect the next 60 pieces of data. That major external event may have made my validity problematic. So controlling for as many things as possible is what we're looking for in internal validity. Drop the word help in the comments and I'll send you a link to my free dissertation help community.
