data entry error skewing prius' average?
#11
04-08-2007, 02:52 PM
Re: data entry error skewing prius' average?
I was curious, so I copied the summary page (the page with one record per car) into a spreadsheet and calculated the totals.
For each car, I calculated the gallons by dividing miles by miles per gallon.
Then I calculate the overall miles, and overall gallons, and divide the former by the latter to get the average mpg over all cars. The result is 36.19 mpg. If I exclude the last record, the average mpg rises to 48.2, which is a lot closer to the published average.
So it would seem that the bad record is not contributing to the overall average.
For each car, I calculated the gallons by dividing miles by miles per gallon.
Then I calculate the overall miles, and overall gallons, and divide the former by the latter to get the average mpg over all cars. The result is 36.19 mpg. If I exclude the last record, the average mpg rises to 48.2, which is a lot closer to the published average.
So it would seem that the bad record is not contributing to the overall average.
Bob Wilson
#12
04-08-2007, 03:19 PM
Re: data entry error skewing prius' average?
Yes, since that was already stated, I didn't bother to repeat it. But since I have this data sitting in the spreadsheet I figure I'd take it a step further and throw out the top and bottom quartiles. The average now comes to 47.49.
#13
04-18-2007, 03:41 PM
Re: data entry error skewing prius' average?
If I understood Jason's post, the mileage figure used is the median of all the mileage figures listed -- which means half are higher and half are lower. It also means that one incorrect figure will have almost no impact on a calculation containing several dozen data points
This wouldn't be the case if the mileage figure was the mean (or average) of all of the mileage figures submitted. When calculating the mean (adding all of the mileage figures and dividing by the number of figures) a grossly inflated or deflated number can indeed seriously skew the outcome.
This wouldn't be the case if the mileage figure was the mean (or average) of all of the mileage figures submitted. When calculating the mean (adding all of the mileage figures and dividing by the number of figures) a grossly inflated or deflated number can indeed seriously skew the outcome.
#14
04-18-2007, 04:50 PM
Re: data entry error skewing prius' average?
Hope the following settles some things - probably will stir up more:
The database does pull the median (50% tile) of the owners averages. I ran a stats test in Minitab using the dbase. The results do show that there is a statistically significant difference in the median mpgs posted. Prius is higher by +1.40 mpg with a confidence interval of +0.90 to +2.00 mpg. Is this a practical significance - probably not. This dbase is a subset of hybrid owners who have found this site and have posted their mileage data.
Both vehicles seem to suck a little (gas), though the Prius appears to suck less.
------------------
Mann-Whitney Test and CI: MPG Prius, MPG HCH
N Median
MPG Prius 1155 47.500
MPG HCH 492 46.050
Point estimate for ETA1-ETA2 is 1.400
95.0 Percent CI for ETA1-ETA2 is (0.900,2.000)
W = 998602.0
Test of ETA1 = ETA2 vs ETA1 not = ETA2 is significant at 0.0000
The test is significant at 0.0000 (adjusted for ties)
----------------
Also interesting is the fact that there are 20 Priuses at 47.5 today (of the 1155), so the median may be pretty insensitive to changes at high/low extremes - even new ones.
The database does pull the median (50% tile) of the owners averages. I ran a stats test in Minitab using the dbase. The results do show that there is a statistically significant difference in the median mpgs posted. Prius is higher by +1.40 mpg with a confidence interval of +0.90 to +2.00 mpg. Is this a practical significance - probably not. This dbase is a subset of hybrid owners who have found this site and have posted their mileage data.
Both vehicles seem to suck a little (gas), though the Prius appears to suck less.
------------------
Mann-Whitney Test and CI: MPG Prius, MPG HCH
N Median
MPG Prius 1155 47.500
MPG HCH 492 46.050
Point estimate for ETA1-ETA2 is 1.400
95.0 Percent CI for ETA1-ETA2 is (0.900,2.000)
W = 998602.0
Test of ETA1 = ETA2 vs ETA1 not = ETA2 is significant at 0.0000
The test is significant at 0.0000 (adjusted for ties)
----------------
Also interesting is the fact that there are 20 Priuses at 47.5 today (of the 1155), so the median may be pretty insensitive to changes at high/low extremes - even new ones.
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