[R] Help With ANOVA (corrected please ignore last email)

[Joshua Wiley]

Hi Amit,

When I copy in your data and run

aov(Intensity ~ Group, data = zzzanova)

I get neither the p-value you showed nor the one you expected.  My
suggestions at things to look at would be

1) Where/How did you get the expected p-value?  Another statistics
program (e.g., SPSS or SAS)?  It helps to know where the expected
value came from.  If it was from another program, I would also
recommend searching the R-help archives (for instance for SPSS and
ANOVA)

2) Perhaps something is happening with the unbalanced design (e.g.,
aov() in R handles it differently than you expected)?

3) You only mention that the p-values do not match.  What about other
aspects?  Are the df the same?  I would try to be certain that the
data in R is the same as what was used to calculate the expected
p-value.  summary(zzzanova) will return some nice summary statistics
for each column of your dataframe.

Cheers,


Josh



On Tue, Jul 6, 2010 at 6:12 AM, Amit Patel <amitrhelp at yahoo.co.uk> wrote:
> Sorry i had a misprint in the appendix code in the last email
>
>
> Hi I needed some help with ANOVA
>
> I have a problem with My ANOVA
> analysis. I have a dataset with a known ANOVA p-value, however I can
> not seem to re-create it in R.
>
> I have created a list (zzzanova) which contains
> 1)Intensity Values
> 2)Group Number (6 Different Groups)
> 3)Sample Number (54 different samples)
> this is created by the script in Appendix 1
>
> I then conduct ANOVA with the command
>> zzz.aov <- aov(Intensity ~ Group, data = zzzanova)
>
> I get a p-value of
> Pr(>F)1
> 0.9483218
>
> The
> expected p-value is 0.00490 so I feel I maybe using ANOVA incorrectly
> or have put in a wrong formula. I am trying to do an ANOVA analysis
> across all 6 Groups. Is there something wrong with my formula. But I think I
> have made a mistake in the formula rather than anything else.
>
>
>
>
> APPENDIX 1
>
> datalist <- c(-4.60517, -4.60517, -4.60517, -4.60517, -4.60517, -4.60517, -4.60517, 3.003749, -4.60517,
>    2.045314, 2.482557, -4.60517, -4.60517, -4.60517, -4.60517, 1.592743, -4.60517,
>    -4.60517, 0.91328, -4.60517, -4.60517, 1.827744, 2.457795, 0.355075, -4.60517, 2.39127,
>    2.016987, 2.319903, 1.146683, -4.60517, -4.60517, -4.60517, 1.846162, -4.60517, 2.121427, 1.973118,
>    -4.60517, 2.251568, -4.60517, 2.270724, 0.70338, 0.963816, -4.60517,  0.023703, -4.60517,
>    2.043382, 1.070586, 2.768289, 1.085169, 0.959334, -0.02428, -4.60517, 1.371895, 1.533227)
>
> "zzzanova" <-
> structure(list(Intensity = datalist,
> Group = structure(c(1,1,1,1,1,1,1,1,1,
>         2,2,2,2,2,2,2,2,
>         3,3,3,3,3,3,3,3,3,
>         4,4,4,4,4,4,4,4,4,4,
>         5,5,5,5,5,5,5,5,5,
>         6,6,6,6,6,6,6,6,6), .Label = c("Group1", "Group2", "Group3", "Group4", "Group5", "Group6"), class = "factor"),
>    Sample = structure(c( 1, 2, 3, 4, 5, 6, 7, 8, 9,
>    10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
>    20, 21, 22, 23, 24, 25, 26, 27, 28, 29,
>    30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,41,42,43,44,45,46,47,48,49,50,51,52,53,54)
> ))
> , .Names = c("Intensity",
> "Group", "Sample"), row.names =
> c("1", "2", "3", "4", "5", "6", "7", "8", "9", "10",
> "11", "12", "13", "14", "15", "16", "17", "18", "19", "20",
> "21", "22", "23", "24", "25", "26", "27", "28", "29", "30",
> "31", "32", "33", "34", "35", "36", "37", "38", "39", "40",
> "41", "42", "43", "44", "45", "46", "47", "48", "49", "50",
> "51", "52", "53", "54"),class = "data.frame")
>
>
>
>
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>



-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/