The scoring method details are not published by the Department of Education due to complexities of calculations involved. However, it is understood that the score is based on a Z-score method which uses Statistical Data. The distribution scores of a large number of students (in thousands) is plotted, which forms a bell-curve. A bell curve is a graph, which depicts the normal distribution. The number of students on the left and right ends is less and it increases towards the centre i.e. you would expect maximum students in the average zone.
The aim of Z-score method is to separate students relative to the performance of other students. Thus, the top-performing students will score the highest scores and will fall on the extreme right of the bell curve with high Z scores.
Z-score is calculated using the raw score, average score and the standard deviation. Z score = (Raw Score - Average) / Standard Deviation. Thus, depending on the Raw Score, Z score can be positive or negative.
Z-score for each test is calculated and converted to a Standard score as follows:
E.g. John scores 24 out of 35 in Quantitative Reasoning. The Average Score of 7000 students is 14.23 and Standard Deviation is 1.65.
Z score = (Raw Score - Average) / Standard Deviation = (24 - 14.23)/1.65 = 5.92. An assumed value of 50 is added to Z-score to calculate standard score. So, the standard score = 5.92 + 50 = 55.92. This is the Standard Score for Quantitative Reasoning.
If Josh has standard scores of 59.85, 73.27, 49.12 for Reading, Writing and Abstract Reasoning respectively, his total score would be a summation of four subjects. This means, the GATE Total Standard Score would be the sum of all 4 tests: 55.92 + 59.85 + 73.27+ 49.12 = 238.16.
If a student hypothetically gets Raw Score = Average score in all four tests, the Z-score would be zero for each test but his Standard Score for each subject would be 50. Therefore, his overall score would be 50 x 4 = 200.
Now, why would the score go below 50? If the Raw Score is below Average, Z-score will be negative. For e.g Josh scored 25 in Analytical Reasoning and the Average was 27.12 with standard deviation 2.71, his Z-score would be (25-27.12)/2.71 = -0.78. This implies, his Standard Score is 50-0.78 = 49.12.