Internal validity would be results of the study that are evidenced based on reality and not on supposition. External is more broad and can be generalized. An example of internal validity would be a research study with a 100 group sample of diabetics that trialed a new diabetic sock and found it to be comfortable with minimal skin excoriation during frequent walking. If they participants were broad in age, with the same type of Diabetes, on the same medication,w with the same moderate to active lifestyle, then you draw a positive correlation from the study and the internal validity would be true.
If you took that same study and broadened the age group with a mix of type 1 and type 2 Diabetics with different lifestyles, then your conclusion may differ and may lead to a generalization.
In designing research study it is important to identify your target population (those who will participate in the study, usually the population that would most benefit from the outcome of the study). This is usually accomplished by identifying the group of individuals with the least amount of variables that will benefit the most from your results. The study ...