Re: [matplotlib-devel] boxplot notch

[Steve.M <sm...@la...>]

I also ran into this problem recently and was disappointed to find that the
notch was based on a normal approximation.
While there are a number of ways to calculate the notch size, it would be
useful to allow the user to supply (either as an optional keyword, or as a
vector input for the notch keyword) their own notch locations. 

For example, I have some code that calculates bootstrapped confidence
intervals - in the case of a significantly non-normal distribution this
would be a better way to find the notch boundaries (which will likely not
even be symmetric). While I'm not advocating building other calculations in,
having the option to supply my own notch locations would be immensely
useful. The default should probably remain as is (IMO) but should also be
mentioned in the documentation as being based on that assumption.
I'm happy to submit an update to do just that if it's seen as a good idea.

Steve.



Andrew Straw wrote:
> 
> Andrew Straw wrote:
>> Also, I think that formula is only for normally distributed data. Which, 
>> especially if you're using boxplots, medians, and quartiles, may not be 
>> a valid assumption.
>>
>> Maybe we should at least raise a warning when someone uses notch=1. The 
>> current implementation seems dubious, at best, IMO.
>>   
> 
> (I sent the previous version of this email a bit too early -- this is
> slightly edited for clarity.)
> 
> I read the following reference:
> 
> McGill, R., Tukey, J.W., and Larsen, W.A. (1978) "Variations of
> Boxplots", The American Statistician, 32:12-16.
> 
> McGill et al. have an entire section devoted to "Choice of Notch Size",
> starting with:
> 
> "In notched box plots, one is, of course, faced with the question of how
> best to determine the widths of the notches. Many methods, both
> classical and non-parametric, might be considered. None will likely be
> best in all cases."
> 
> ...
> 
> 

-- 
View this message in context: http://old.nabble.com/boxplot-notch-tp26798967p27249739.html
Sent from the matplotlib - devel mailing list archive at Nabble.com.