Here is it which they once showed on STAR NEWS
First one Box type Gadget is attached to the TV in some homes (randomly) of some big cities (B'bay, Delhi, Culcutta, Banglore…etc.) for at least 1 week.
Every time the viewers switch on the TV or view a serial /show they have to press a button on the Gadget so that the program they view gets recorded on it.
After completion of the complete period (1 week or so) all the Gadgets are collected & the recordings are viewed to know which serial had been watched & how many times.
The most watched serial gets more points & the less watched one gets less points.
This is how the TRPs are calculated
So the Bottom Line is
The TRPs do not depict the true picture of the viewers of the show.
Only the viewers of Big cities are considered (that too not all)
The viewers of small cities, towns & villages are neglected (who, I think contribute the most to the TV viewers)
Also many people who watch the high TRP serials also think that the serial is stretched too much & has too much of melodrama, extra marital affairs etc. But they watch it only because they have been watching it from
So Nimmo getting low TRP does not mean it has very less viewers.