The 90/10 rule

I’m about to suggest that you study less, and not more, for your exams. Shocking, I know, but bear with me. This is the kind of blog post that will give your teacher nightmares.

Some years ago I came across an unusual and intriguing study theory: the 90/10 rule. This theory is not in The Holy Grail of Exam Success, as I was keen to avoid its being construed in the wrong way. Therefore, take it with a pinch of salt, but equally profit from it where you can.

The ‘ninety-ninety rule’ (or 90/10 rule as we shall call it for our purposes) is attributed to Dr Tom Cargill who, as a programmer and software developer at Bell Labs in the 1980s, described the rule as follows: “The first 90% of the code accounts for the first 90% of the development time. The remaining 10% of the code accounts for the other 90% of the development time.” This sound bite, while amusing, was intended to highlight how frequently software development projects overrun their schedules.

So what has a software developer’s humour got to do with exams I hear you ask? More than you might think.

In the context of studying for exams, the 90/10 rule can be thought of like this: it will take you the same amount of time to learn the most difficult final 10% of a subject, as it will the easier first 90%.

This is, of course, an obvious generalisation, and the actual percentages are merely arbitrary and could vary significantly, as they do with similar theories such as the Pareto principle and others. Nevertheless, the gist of the rule is sound: if you were to try to learn absolutely everything in a particular exam subject, it would likely take you much longer to understand, learn and remember the most complex details of that subject (the icing on the cake, if you will, the final 10%), when compared in relative terms to the time that it would take you to absorb all the rest of the information which makes up that subject (the other 90%).

So, am I suggesting that you don’t study what you consider to be the most difficult topic(s) in a subject? Absolutely not – this kind of thinking would be exam suicide, especially if the entire exam could be described as one big difficult topic! However, I do want to encourage you to be selective and logical with the time that you spend studying. Now let me give you a real-world example.

In my final year law exams, I had to sit a business law exam. I understood most of the topics in this subject well, except for one: corporate taxation. Like many of my classmates, I found this topic complicated and frustrating. I couldn’t easily remember the multitude of rules and the numbers never seemed to add up. When doing practice papers, I always found that I would waste far too much time on this type of question, to the detriment of the rest of my answers. Therefore, I decided to apply the 90/10 rule. Since corporate taxation was a stand-alone question topic which would likely only ever be worth a maximum of around 12 marks (according to my careful analysis of past papers), I figured that if I answered the rest of the exam well, I could just about afford to ignore the corporate taxation question altogether. In the end I didn’t study for it at all, and instead focused all my attention on improving my knowledge elsewhere.

On the day of the exam, sure enough, there was the corporate taxation question. I smiled to myself, noting that it was worth 10 marks out of 100 overall, a perfect example of the 90/10 rule in action. I ignored the question, sticking to my gameplan. This meant that I now had an extra 25 minutes or so with reading time factored in to spend on the rest of the paper (since this first paper was worth 80 marks and was three hours long). I used this extra time wisely, and completed the paper to the best of my abilities.

When I received my marks some months later, I found to my delight that I had achieved 77% overall, a solid distinction grade. The 90/10 rule had paid off, big time. This just goes to show how by carefully allocating your study time, taking educated risks and having an awareness of your own limitations, you can sway the odds in your favour. Few students can ever achieve 100% in an exam, but even fewer have taken the time to realize that you don’t have to and that you’re probably not expected to either. If you simply cannot understand a topic, it isn’t integral to any other topics and isn’t worth that many marks, then stop trying to flog a dead horse! Just ignore it.