Comments
Paragraphs lack coherence. The paragraphs do not flow with same ideas as they should. If you look at your last paragraph, it has started with a description of what validity is, then the next sentence brought out the different types of validity. This is too much squeezing and the work should be more organized so that related ideas are brought out in the same paragraph and avoid scattering of ideas to avoid confusion. You did not tackle the question according to the instruction. The question had clearly stated that you were to use one of the case examples in chapter 5, but all you did was to explain deeply what external validity means and its challenges.
There is a lack of order in your work. Your work lack order on how the points should be arranged, for example, you start your essay by giving challenges of external validity and later defining validity on your last paragraph in the first line. This could have been made better if the definitions came on the first paragraph and then the limitations came on the last one. There are a lot of repetitions in your essay. Many phrases have been unnecessarily repeated such as in the second paragraph where almost a full sentence has been repeated. This makes the uninteresting to read. You should proofread your work before submission to avoid such errors.
You clearly mentioned all aspects of validity. Although you just listed them, you clearly bring out different aspects of validity and different factors that can make a sample be taken as valid to act as a representation of the whole unit under research. You have raised important ideas and given them enough support. Your explanation was complex especially on the first paragraph where the challenges have been clearly explained giving instances on when it is unrealistic to generalize a study.