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Friday, October 12, 2007

Chat Transcript 10/12

Thanks to everyone who participated in the chat today. You can post any other questions here and I'll do my best.


Anonymous said...

Do you know how pfx and pfz are calculated? In other words, do you know the formula (if there is one) used to calculate those metrics? I would have thought it would be equal to .5*a*t^2 with "a" being az for pfz and ax for pfx, but that is not the case: using that method the magnitude of the calculated values are too high.

Now just a comment... I liked your last article on Using the temperatures you posted for the stadiums, and elevations I found, I calculated air densities in kg/cubic-meter for those stadiums (using the dry air constant) and listed them below. The difference between completely dry and saturated air is about a 1% decrease in air density. The difference between 1025 and 1000 millibar conditions (sea-level pressures) is about a 2% decrease. The difference between freezing temps and 95 degree heat is a 13 or so % decrease. So temperature and elevation is what matters most. Your Pct. Speed Difference table is sort of in the same order of my listing, but there's plenty of stadiums that are on complete other sides of the table. I know for a certainty at least some of the time Comerica's start speed has been 2-3 mph slower than radar guns on TV with end speeds being rather reasonable.
at&t 1.210421548
mcafee 1.205955856
safeco 1.203235236
petco 1.196216755
tropicana 1.193750955
minutemaid 1.190648317
fenway 1.190051602
shea 1.189922509
yankee 1.189707379
citizens bank 1.178579127
anahiem 1.170748152
metrodome 1.165644624
comerica 1.164838215
jacobs 1.164781009
cellular 1.164539825
dolphin 1.164091253
dodger 1.164032862
miller 1.162786252
wrigley 1.162659604
pnc 1.161822595
busch 1.149111808
gabp 1.146446619
kauffman 1.138447675
rangers 1.137247584
turner 1.129902359
chase 1.129572072
coors 0.97579623

Dan Agonistes said...

I'm not sure I follow your first question. pFX is calculated as the square root of the sum of pfx_x squared plus pfx_z squared. I don't know what you mean by pfz...

Thanks for the putting this list together. This is exactly what I wanted to do but didn't get the elevations for the parks. The correlation between these values and my pct diff in the article was .26 when you leave Comerica and Dolphin Stadium out of the mix. I definitely think there is something not quite right about the setups in those parks.

Thanks again.

Anonymous said...

I'm sorry about that. By pfx I meant pfx_x and by pfz I meant pfx_z. And to be clear the "t" in .5*a*t^2 is the time flight of the ball (about .4-.5 seconds) calculated from the equation 1.417 = y0 + vy0*t + .5*ay*t^2.

Dan Agonistes said...

pfx_x and pfx_z are calculated from the acceleration data included in the XML data. You can read more about that at