How is the PSLE T-score calculated without MTL?

Before you go and get your exemption for Mother Tongue, you may want to ask the following questions:

How will the PSLE Aggregate score be calculated without mother tongue language?

Will my child be advantaged or disadvantaged by not having MTL as a subject?


First of all, you need to know what the PSLE Aggregate score actually is.  Here's a simple explanation:

The PSLE Aggregate score is the sum of a student's T-scores across 4 subjects.

What is a T-score?

The Transformed Score or T-score is the mark a student receives AFTER the raw score (what the student got in their exam) it is "transformed"or "standardised" to show the relative position of a students’s performance as compared to the performance of all the other students in that subject.

So how is the T-score calculated for each subject?

T= 50+10(X-Y)
                      Z
             
X: student’s mark for the subject
Y: average mark (mean) scored by all students
Z: standard deviation

Here are some examples with different raw scores:

Example:
• Pupil’s mark (X) in exam
• Average mark (Y) scored by all students
• Standard deviation (Z)

Example A

T = 50 + 10 (50 – 63.5)
                             9.4
   = 35.64

Example B

T = 50 + 10 (100 – 63.5)
                              9.4
   = 88.83

Poor student.  Scored 100%, but look at his T-score.

Example C

T = 50 + 10 (63.5 – 63.5)
                              9.4
   = 50

Note that in any T-score calculation the forced average is always 50.  If a pupil scores the cohort average, no matter how high or high low that number is, the T-score will always be 50.  Thus the average PSLE score will always be 200).

A note on Standard Deviation.  The SD refers to the score within 68% of the average (using a normal distribution).  So if the SD is 10 that means 68% of the cohort scored within 10 marks of the average.  If the SD is 5 it means that 68% of the cohort scored within 5 marks of the average and so on.

The higher the standard deviation (spread of marks within 68% of the mean), the higher the score.

PSLE AGGREGATE SCORE WITHOUT MTL

MOE doesn't actually give a clear explanation as to how the PSLE Aggregate Score is derived for a student who is exempted from MTL.  They just say the student will neither be advantaged nor disadvantaged.

However, after checking around, popular belief is that the average of the student's other 3 subjects will be used to calculate their T-score for MTL.  So, if your child scores well for Maths, English and Science, then your child will be at an advantage.  Assuming that the T-scores for the 3 subjects are simply averaged.  However, if the raw scores are averaged and applied to the formula, then it still depends on how well the cohort does for MTL.  A low cohort MTL average would mean a MTL exempt student with 3 good subject average scores will score higher and be at an advantage.  Conversely, a high cohort average for MTL would mean a lower score, and thus no advantage.

So ideally, the average score of the 3 subjects used to calculate the MTL score should ideally be above the raw average of the MTL cohort and the SD should be as large as possible (which is unlikely as SDs do not tend to vary from year to year).