'mean' and 'sd' calculations do not match
Ulrich Leopold <uleopold at science.uva.nl> writes:
Dear list, I am using R 2.1.1 on a Fedora 3 Linux, 32 bit PC. If I compute the aggregated mean and the standard deviation I get standard deviation values for factors where the mean was not computed. It seems to me that this is somehow related to the NA values. But I don't quite understand what is going wrong?
You're using na.rm=TRUE on the sd calculation, but not on the means! (The NA's generated for sd are likely groups with only one observation).
Could it be related to the data import already? Some of the imported data got the character strings NA and others <NA>. But they are defined from the same values, -9999.
No. It signifies a problem, but not this one. The <NA> is used for factor and character columns. Most likely (can't think of any other reason) some of your data are not numeric - "," instead of "." and similar typos will do that to you.
I used the code below. Below the code are parts of the results.
Cheers, Ulrich
Data import:
chemicS <- read.table("ChemieUlli_4_Quellen.csv", header = TRUE, sep =
",",na.strings = "-9999")
Count EC NO3 NO2 NH4
3504 630.0000 33.00 0.001 0.01
3505 NA 26.66 <NA> <NA>
3506 NA 0.72 <NA> <NA>
3507 NA NA <NA> <NA>
3508 NA NA <NA> <NA>
3509 NA NA <NA> <NA>
3510 1210.0000 14.00 0.001 0.01
3511 1265.0000 12.00 0.001 0.01
3512 1400.0000 14.00 0.001 0.01
3513 1427.0000 12.00 0.001 0.01
3514 1410.0000 7.00 0 0
3515 1520.0000 8.00 0.001 0.01
3516 1470.0000 7.60 0 0
3517 1170.0000 10.00 0.001 0.01
3518 4570.0000 20.00 0.001 0.45
3519 8560.0000 0.50 0.14 0.31
3520 708.0000 39.00 0.001 0.01
3521 833.0000 40.00 0.01 0.01
3522 NA NA <NA> <NA>
Computing the mean:
aggregate(chemicS$EC, by = list(east=chemicS$EST, north=chemicS$NORD),
FUN = mean)
Count east north Mean
350 89885 103160 318.50000
351 55870 103510 400.00000
352 82570 104845 637.33333
353 79119 107433 NA
354 79160 107462 362.77778
355 83010 108990 NA
356 82810 109010 NA
357 69135 112992 NA
358 55490 120140 142.25000
359 56580 120600 NA
360 56582 120607 NA
361 58050 125350 NA
362 58059 125360 NA
363 60360 128191 NA
364 65448 128293 252.50000
365 65472.5 128308.1 NA
366 61412 131141 NA
Computing the standard deviation:
aggregate(chemicS$EC, by = list(east=chemicS$EST, north=chemicS$NORD),
FUN = sd, na.rm = TRUE)
Count east north Stdev.
350 89885 103160 4.9497475
351 55870 103510 NA
352 82570 104845 19.6553640
353 79119 107433 NA
354 79160 107462 73.6745848
355 83010 108990 NA
356 82810 109010 15.6950098
357 69135 112992 NA
358 55490 120140 5.3150729
359 56580 120600 NA
360 56582 120607 22.4435801
361 58050 125350 NA
362 58059 125360 23.3108523
363 60360 128191 20.9789577
364 65448 128293 10.6066017
365 65472.5 128308.1 NA
366 61412 131141 8.6184556
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