Mirror Magazine

Mirror Magazine

Lost in Z score
Trying to understand how to work out the Z score may feel like a tedious job. Here, Natasha Fernandopulle gives you a guide to simplify matters
The Z Score… Yes, many a question was raised when it was first introduced in Sri Lanka. The new system was introduced in the year 2001, but do you really know what it is all about? We have attempted therefore to explain to you briefly what the Z Score really is about.

Changes in the education structure
Changes in the structure of the education system, in Sri Lanka came into being around the year 1999, and was fully implemented in the year 2000. The change occurred from grades one to 13.

Before the year 2000, students sitting for the G.C.E. Advanced Level examination offered four subjects, but with the new education reforms, the system was changed to students having to offer three subjects and sit for a Common General Test.

Students under the old system, or those who had to repeat were allowed to offer four subjects in the years 2000 and 2001. After the year 2001, all students had to offer three subjects along with the Common General Test.

The problem
The issue at this point was that there were two groups of students, those who had offered four subjects and those who had offered three subjects. They in turn were to enter university on a very competitive basis, to read for the same courses of study.

The usual system in the selection of students for university was the average of the raw marks. This method may be used if the distribution of marks of the different subjects are the same.

But with the two sets (three and four subjects), this was not the best option due to many reasons. For instance, it had to be assumed that the examination papers had similar levels of difficulty. Another reason was that when comparing the three and four subject groups, it had to be assumed that those sitting the three subjects, sat for a fourth subject and got a mark equal to the average of marks obtained in their three subjects. This too was not a valid assumption.

The use of raw marks
This system was used in Sri Lanka for many years, but is not widely used in countries, which are educationally advanced in terms of policy, in order to select students for university. According to ‘Selection of Students to Universities from 2001’, a publication by the University Grants Commission, it is stated that studies have shown, that when using the aggregate of the raw marks in selecting students to university, “undue competitive advantage may accrue to some students by virtue of the subjects they offer.” This is because examination papers in all subjects are not of the same level of difficulty. In short, some subjects are considered easier than others. It has also been observed that students score high marks in some subjects than in others. As a result some students have more of an advantage over others.

Take this as an example – say that one possible combination for a student opting to read for a degree in biological science, is to offer biology, chemistry and physics for the

A/ Level examination and another combination is to offer biology, chemistry and agriculture. Speaking to M.M.P. Premakumara, Senior Assistant Secretary, University Admissions, University Grants Commission, he said that, more students would score higher marks in agriculture than in physics. This gives some students more of an edge over others, and in turn gives them a higher chance of entering university.

Such a scenario was “considered unfair and undesirable,” according to the booklet published by the University Grants Commission, ‘Selection of Students to Universities From 2001’.

The solution
Therefore, the University Grants Commission had to find a suitable formula to take both groups into account. This is where the ‘Z score’ came in. It is said to be a universally accepted solution, which converts the raw marks gained by the students in different subjects to ‘Z scores’, using a standard scale, called, the Z distribution.

Let us attempt to clarify what this means in simpler terms. Take for example the currencies in different countries. They do not have the same value, like the Sri Lankan rupee and the Singapore dollar, for example. Thus, for the purpose of exchange their values are expressed through the American dollar.

Now, you may be able to understand the Z score a little better. So, what it does is, it converts the raw marks of different subjects to ‘Z scores’, thus making the university entry value of different subjects fall within one scale, making it easier to compare the approximate value of each subject.

About the Z distribution
One has to know elementary statistics in order to understand the theoretical basis of the conversion of raw marks. Again, with reference to the ‘Selection of Students to Universities from 2001’, what is done is to convert the raw marks in different subjects to a scale, which has “a common mean and standard deviation,” which is a measure of the spread of marks in a given subject. Once again getting into statistics, the Z distribution, has a mean of zero (0) and a standard deviation of one (1).

So, to convert the raw marks into Z scores, the raw marks, which the student has obtained for a particular subject, is subtracted by the mean mark of that subject. This is then divided by the standard deviation of marks for that particular subject.

This is basically what the much controversial Z Score is. A few more details

1. The selection of students depends on rank order, on the average Z scores obtained

2. The Z scores are made available for the students

3. One has to obtain at least ‘S’ Grades in all three approved subjects and gain a minimum of 30% for the Common General Test paper

4. For Arts courses (Arts, Arts (SP), Arts (SAB), Peace & Conflict Resolution), Ayurveda, Unani and Siddha Medicine, admission to courses will be on an all-island merit basis

5. Apart from the courses mentioned above, for all other courses, admission will depend on a dual criteria, namely, all-island merit basis and merit on district basis

6. Up to 40% of the places available will be under all island merit criteria and 60% on the district merit criteria

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