[R] How can I save the result for goodness of fit test
Michael Friendly
friendly at yorku.ca
Wed Mar 17 14:22:45 CET 2010
pinusan wrote:
> Dear All,
> I run the goodness of fit test using goodfit() in vcd package.
> The result is as follow:
>
> Goodness-of-fit test for poisson distribution
>
> X^2 df P(> X^2)
> Pearson 1.053348 2 0.5905661
> Warning message:
> In summary.goodfit(gf) : Chi-squared approximation may be incorrect
>
> I want to save the the test statistics(X^2), df, and p-value. How can I save
> the result. Actually, I want to make a table.
The usual general answer to this question is to read the documentation,
and look for the Value section. ?goodfit gives
Value
A list of class "goodfit" with elements:
observed observed frequencies.
count corresponding counts.
fitted expected frequencies (fitted by ML).
type a character string indicating the distribution fitted.
method a character string indicating the fitting method (can be either
"ML", "MinChisq" or "fixed" if the parameters were specified).
df degrees of freedom.
par a named list of the (estimated) distribution parameters.
So, the quantities you want are not returned in the resulting object.
But, they *are* returned from the summary method that you used to print
the result. [That isn't documented, so perhaps that could be added.]
So, the 2nd general answer to your question is to use str() on the
result of the summary() function.
> ## Simulated data examples:
> dummy <- rnbinom(200, size = 1.5, prob = 0.8)
> gf <- goodfit(dummy, type = "nbinomial", method = "MinChisq")
> summary(gf)
Goodness-of-fit test for nbinomial distribution
X^2 df P(> X^2)
Pearson 0.5865029 1 0.4437746
Warning message:
In summary.goodfit(gf) : Chi-squared approximation may be incorrect
> result <- summary(gf)
Goodness-of-fit test for nbinomial distribution
X^2 df P(> X^2)
Pearson 0.5865029 1 0.4437746
Warning message:
In summary.goodfit(gf) : Chi-squared approximation may be incorrect
> str(result)
num [1, 1:3] 0.587 1 0.444
- attr(*, "dimnames")=List of 2
..$ : chr "Pearson"
..$ : chr [1:3] "X^2" "df" "P(> X^2)"
>
> result
X^2 df P(> X^2)
Pearson 0.5865029 1 0.4437746
>
So, there you go!
> In addition, there is warning message "In summary.goodfit(gf) : Chi-squared
> approximation may be incorrect".
> How can I interpret this result.
>
Read the Details section
> Have a nice day.
>
--
Michael Friendly Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street http://www.math.yorku.ca/SCS/friendly.html
Toronto, ONT M3J 1P3 CANADA
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