As you say it is an indicator not a guaranty, and a really good one when you look at a long enough history. a family crisis during my degree which several impac

As you say it is an indicator not a guaranty, and a really good one when you look at a long enough history. a family crisis during my degree which several impacted my performance I would say that it is a pretty good indicator of how you behave in the future if those circumstances happen again. What is dangerous is not using enough data to make the determination. Your friends A-levels is just one data point, making a judgement on one data point isn't going to make a good choice.