###### COMMENTARY

10 question papers with 50 questions each. These 50 questions were taken randomly from the original 100 questions in one p aper. (We repeated the whole exercise with the other paper with no substantial change in the results.) In this way now, we had 11 test papers for each candidate – one original one and 10 ﬁctitious ones.

Since the ﬁctitious papers were made from the original test paper, the marks of

###### Figure 1: Ranking Noise (Paper 1) with Errors

Original ranks 50000

40000

30000 Sampled ranks with errors

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0 100 90 80 70 60 50 40 30 20 10 0

Marks

###### Figure 2: Ranking Noise (Paper 1) with Errors

3500

3000 Sampled ranks with errors

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Original ranks 0 95 90 85 80 75 70 65 60 Marks

Figure 3: Histogram of Ranks in 10 Sample Papers for Students Having 68 Marks in Original Paper

350 300 250 200 150 100

50 0 250 500 750 1250 1750 2250 2750 3250 3750 4000

1000 1500 2000 2500 3000 3500

###### Figure 4: Ranking Noise (Paper 2)

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Sampled ranks with errors 3000

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1000 Original ranks 0 95 90 85 80 75 70 65 60 Marks

each candidate in each of the 10 ﬁctitious papers were known. Thus each candidate had 11 marks and ranks – one original and 10 in the ﬁctitious papers. In fact, there are several students associated with each original rank and each of these students has 10 other ﬁctitious ranks.

We plot the original data on a graph, i e, make a plot of the ranks in the original data vs the corresponding marks. This curve

is shown as the unbroken line in Figure 1.

At each rank, we now take the number of students who got that rank and ﬁnd their ranks and marks in the 10 ﬁctitious papers. Typically, there are around a 100 students at most ranks and remembering that each of these students has 10 other ranks, we get a large enough sample to warrant a statistical interpretation. We take this distribution and compute its mean and in the original Paper 1), we take the 164 students who have got this rank. For these 164 students, we consider their ranks in each of the 10 ﬁctitious papers. Thus we have in all 1,640 ranks in the ﬁctitious p apers and ﬁnd their mean and the standard deviation of this distribution. The distri bution is shown in Figure 3.

It is interesting to see the same graphs for Paper 2 which we exhibit as Figure 4. The second paper shows a trend similar to the ﬁrst paper. In detail however the number of candidates at each rank is more than in the Paper 1. The curve therefore rises more steeply and the standard errors are also a little higher.

The detailed graph of the sample in this case is shown for the rank 968 (73 marks in Paper 2) as Figure 5 (p 23). The number of candidates at this rank in this paper is

185. This histograms presented is therefore for 1,850 ranks. The mode of the distribution is near 1000 and the quartiles are near 550 and 1,575.

We have performed this analysis with a set of 10 sample papers. To see if the trend is dependent on the sample size (number of papers) we repeated the analysis for a sample of 50 papers and we ﬁnd

standard deviation, a very similar histogram. This is shown

which gives us an estimate of the spread around the mean.

This information is plotted in Figure 1 as the broken line. The data points are the mean of the distribution at each rank and the error bars are the standard deviation.

To make the data more obvious, the same plot for the ﬁrst 30 distinct ranks is shown in Figure 2.

To further study the samples, we choose an arbitrary rank in the original paper. We take a rank near 1,000, assuming that in a typical in Figure 6 (p 23).

From the data, we calculate the number of “students” who are in a given range of marks. For Paper 1, we see that at rank 1,060 in the original paper, there were 164 students. If we take the distribution of their marks in the 10 ﬁctitious papers, we see that roughly 50% of the students lie between rank 758 and 1,521, a relatively large spread in ranks. Similarly, for Paper 2, the rank we have chosen is 968 and in this case the spread for 50% is between ranks 646 and 1,639. It is remarkable that the two papers show a relatively similar spread.

Recall that these are students who got exactly the same marks (rank) in the original test. Furthermore, the ﬁctitious papers that we have made are sampled from the original paper itself. But, even with this, there is indeed a spread in the ranks. What this implies is that the exact rank that a student

examination, rank gets in such an examination is crucially 1,000 would be the dependent on the sample of questions that qualifying rank. At are chosen from the larger set of questions. rank 1,060 (68 marks In our case, this was 50 questions from a

december 20, 2008 EPW Economic & Political Weekly

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