[R] QQplot normally distributed

[Liviu Andronic]

On Tue, Jun 19, 2012 at 12:42 PM, Özgür Asar <oasar at metu.edu.tr> wrote:
> Following a straight line indicates less evidence towards non-normality. But
> QQ-Plot is an exploratory tool.
>

> You can confirm your ideas obtained from the QQ-Plot via noramlity tests
> such as Shapiro-Wilk test.
>
Hmm, some gurus on this list would likely disagree.

> fortune('normality')

Usually (but not always) doing tests of normality reflect a lack of
understanding of the power of rank tests, and an assumption of high power for
the tests (qq plots don't always help with that because of their subjectivity).
When possible it's good to choose a robust method. Also, doing pre-testing for
normality can affect the type I error of the overall analysis.
   -- Frank Harrell
      R-help (April 2005)

> fortune('normality')

The issue really comes down to the fact that the questions: "exactly normal?",
and "normal enough?" are 2 very different questions (with the difference
becoming greater with increased sample size) and while the first is the easier
to answer, the second is generally the more useful one.
   -- Greg Snow (answering a question about a "normality test" suitable for
      large data)
      R-help (April 2009)


For more on this, see ?SnowsPenultimateNormalityTest in TeachingDemos
and the references within. Also search the archives as this topic pops
up regularly.

Regards
Liviu