Thursday, March 15, 2012

Prediction of rank in LDC (Ernakulam) exam


Ever since my wife wrote the LDC (Lower Division Clerk) – Ernakulam district exam and was on tenterhooks regarding the rank, she was always scouring the net for possible position in the rank list. She got 64 marks and the probable list contained 2202 persons. So, I offered her a prediction, based on statistics. It was a very very long time that I’d got anything to do with statistics, so a thorough search on the Net had to be made. The calculation proceeded along the following lines.

Representative sample for proportions n0 = Z2.pq / e2  (Eqn 1)

Where 
n0 = sample size, Z2 = abscissae of normal curve that cuts off an area at the tails (1 – desired confidence level, taken as 95%), 
e = desired level of precision (taken as +/-5%), 
p = estimated proportion of an attribute that is present in the population (taken as 0.5 for maximum variability), 
q = 1-p.

Z = 1.96 for 95% confidence level (from normal curve)

Substituting the values in Eqn 1, n0 = 385

Applying finite population correction, n = n0 / (1 + (n0 – 1)/N) (Eqn 2)

Where n = required sample size, n0 = obtained in previous step and N = population (2202, for this case)

Substituting in Eqn 2, n = 327

The roll numbers of shortlisted candidates were exported to Excel and a macro was run to identify 327 random numbers from the list. Marks obtained by these numbers were collected from Kerala PSC’s website, tabulated in Excel and sorted. My wife’s marks were introduced in the middle of people who obtained exactly 64 marks each. She came at 41st place.

This was for a sample size for 327, extrapolating to the total, 41 x 2202 / 327 = 276.

Applying precision of +/-5%, her rank can be anywhere between 165 to 385. This I say with 95% confidence, though I have a hunch that it will be between 250-300.

I’m writing this on Mar 15, 2012, the results are expected by this month-end. We’ll see then!

31.3.2012

The results have been published. Her rank came out to be 254.
Prediction is 100% successful. Statistics is great!

4 comments:

dr prasanna said...

ranklist vanno ??prediction seriyayo atho .....

dr prasanna said...

wow...great!!!sammathichirikkunnu...

hai said...

hai sir,me too got 64 marks in LDC exam held on 4-1-2014(ekm dst).what will be my rank?pls reply me

Prem P said...

Hallo sir, i got 66 mark in LDC ernakulam exam 2014. Please predict my posssible rank..please please please