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Many schools make the following comparison regarding diploma scores:
which is actually this mismatched comparison:
Instead, one of the following comparisons should be used:
or
We use the all-candidates comparison in our IB Score Reports for two reasons: it makes sense and schools tend to fare better in their comparison against the world.
Many schools take their average diploma score, as reported on their IBIS School Statistics report, and compare it to the worldwide average most cited by the IBO. That's understandable, given that those are the averages readily available to schools. Such a comparison, however, is often mismatched. This article will explain the issue and how we resolve it.
Let's look at an actual example involving May results from a client school.
As the table shows, it appears that the school has outperformed the world by an average of 2.5 diploma points over the last five years.[1]
However...
The school's average diploma score, as presented on their IBIS School Statistics report, is qualified. Specifically, the average is clearly stated to be the...
"Average points obtained by candidates who passed the diploma." [emphasis added]
In contrast, the worldwide average diploma score most cited by the IBO includes all candidates, even those who did not pass the diploma (i.e., were not awarded a diploma).
Thus, the comparison shown in the table above – the comparison made by many schools – is actually between:
Let's relabel Table 1 to make that clear:
Given that the school had candidates who did not earn the diploma because they scored too few points, that's a mismatched comparison.
There's nothing wrong, of course, with a school looking at the average diploma score of their successful candidates. But that average shouldn't be compared to a worldwide average that includes unsuccessful candidates. That's comparing apples and oranges, and doing so inflates the school's performance relative to the world.
Why do we say that the all-candidates version of the worldwide average is the one "most cited" by the IBO?
First, prior to 2021, the only average diploma score published in the IBO Statistical Bulletin was the all-candidates version.[2] Since 2021, the IBO has added the successful-only average (i.e., diploma-awarded average), but it's easily overlooked. In the May 2023 Statistical Bulletin, for example, the all-candidates average appears on pages 3, 5, and 7. The successful-only average appears just once, on page 10.
Secondly, when the IBO has included an average diploma score in their annual news post announcing the release of exam results, it's always been the all-candidates average.[3]
A proper comparison would be either of these:
or
Our own IB Score Reports have always used the first pair of averages, those that include all candidates, both successful and successful.
The primary reason is that it makes intuitive sense to us. When teachers report the average of an assessment given in class, they don't exclude those who earned low marks and did not pass. Similarly, the IBO doesn't remove students who earned a 1, 2, or 3 when calculating the average for individual subjects in their Statistical Bulletin. So why do anything different for diploma scores?
Thus, our IB Score Reports would present the comparison as follows.
The new table shows that the school has outperformed the world by an average of 2.1 points over the last five years, which is a bit lower than before, although certainly still impressive.
Regardless, the comparison is now apt: both the school average and the worldwide average are based on all candidates, both successful and unsuccessful.
The other way to construct a fair comparison would be to use the school average and worldwide average of successful candidates only. Like so:
When the comparison is made between successful candidates only, the table shows that the school outperformed the world in just three of the five years – and actually underperformed in one.
The explanation for this surprising result is simple:
Schools generally have just a few candidates who are not awarded the diploma because of too few points. Removing those candidates raises the school's average, but in most cases, only a small amount. For the school in our example, the difference was just half a point or less, as we can see in the following table:
In contrast, removing unsuccessful candidates tends to raise the worldwide average by about 2 points.
It's worth noting that the difference between the successful-only and all-candidates averages shrank as a result of lenient grading that began in 2020 and continued the next two years. Historically, the difference has averaged around 2.4 points, as we see when looking at 2019 and 2023 in the table above, and the prior five years in the table below:
Thus, excluding unsuccessful candidates from a school's average tends to raise their average a small bit, while excluding unsuccessful candidates from the world average tends to raise the world average by more than 2 points.
We're confident that the vast majority of our client schools fare better when comparing their all-candidates average against the all-candidates world average, rather than comparing their successful-only average against the successful-only world average.
When a school doesn't have any unsuccessful candidates, their all-candidates average will be the same as their successful-only average. In that case, too, it will be best for the school to compare their average to the lower, worldwide all-candidates average than the higher, worldwide successful-only average.
So far, the discussion has only addressed May results. What about November results?
Essentially, everything said about May averages holds for November averages as well.
As the following tables show, the difference between the successful candidates and all candidates in November has averaged around 2.6 points over the past five years. If we look at the prior five years, we see that the difference averaged over 3 points.
One unavoidable consequence of our approach is that "all candidates" includes diploma candidates who received scores in some, but not all subjects.
Admittedly, a better approach would be to include all and only those diploma candidates who have scores for all subjects, plus Theory of Knowledge and their extended essay. That strikes us a reasonable approach, especially for schools with small cohorts, where low-score outliers can significantly affect an average.
Unfortunately, the IBO doesn't disaggregate worldwide diploma scores in that way, and so our choices for the worldwide norm are just "all candidates" (where that really does include candidates who earned 0, 1, 2, etc. total points) or "successful candidates only." For the reasons we've explained, we prefer the former rather than the latter.
1. Small discrepancies may arise due to rounding. For example, 2019 in Table 1 shows, 33.2 − 29.7 = 3.6 (rather than 3.5), because the underlining values were, 33.20 − 29.65 = 3.55. (Return to text)
2. The IBO Statistical Bulletin is a publicly available document of aggregate data and results. (See: IBO DP Statistical Bulletins.) In contrast, the IBIS School Statistics report is a private report of data and results pertaining to an individual school. (Return to text)
3. See, for example, the following news posts (which are listed by the exam session to which they refer rather than publication date):
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