Direct drive vs belt vs rim vs idler arm

Hi Ketchup,
The fewer data points that you have, then the higher the standard deviation which let's say is increased uncertainty. Just the fact that you are going to make two 0.100" pencil marks 0.02" apart is going to add to the measurement error. A few hours of running time will make the marks far enough apart to at least minimize the pencil mark measurement error.
Ok, you have a measurement after 666 revolutions. Go ahead and let it run for a few hours and see if your results are the same. The results might look worse, or they might look better, but I doubt they will be exactly the same. (I've been known to eat my words before). :)

I used to think I was normal until I heard a difference in power cords. Why oh why did that have to happen to me.

Tony,

I'm not totally sure what you're trying to say. There is no problem or error with the pencil marks. You make one mark on the wall at the beginning of the record and you make another mark on the wall at the end of the record and measure between them. If the two marks are 3.25" apart after 20 minutes and your turntable's spindle is 31" from the wall, your total speed error over those 20 minutes is 0.0025%.

I haven't even tried aftermarket power cords. We both use Thiels, solid state power amps, and ARC tube preamps. Can you recommend a "cheap" aftermarket power cord for one of my components that will make me think I'm even more abnormal? Sorry for the diversion. Back to speed stability...

Off the deep end will be when someone here says that a power cord made a difference in turntable speed accuracy. Which is both on- and off-topic.

Hi Ketchup,
I'm not sure how else to say it. If you have a timeline or someone who has a timeline could do that test- measure after 20 minutes and then do a 2 hour measurement. See if the per rotation error calculates out the same or not. That would dispel my notion about too few data points.
I have a MIT powercord on my CD player. It made a difference; but I would not call it a cheap powercord. btw- seems like I saw a thread somewhere once that claimed a better powercord on their VPI controller, maybe, made a difference?